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How long would it take for a medium-sized human caster remaining inside a demiplane after the spell expires to suffocate. Assume no other mass displacing volume in the space.

Per the rules of the demiplane spell (PHB, p. 231):

the demiplane appears to be an empty room 30 feet in each dimension, made of wood or stone. When the spell ends, the door disapears, and any creatures or objects inside the demiplane remain trapped there as the door also disappears from the other side."

(Or, if I am making an incorrect assumption in this question, why wouldn't they suffocate?)

NOTE: Following on some initial comments this is not a question about extra-planar travel to demiplanes in general, but specifically about the demiplane spell and the space it leads to. Question hopefully now clarified.

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There's no way to reason one way or the other about this outside a DM's particular campaign, since this is explicitly the DM's domain to decide and the game provides no defaults.

In the context of the Dungeon Master's Guide chapter on the planes, the DM is given instructions explicitly and repeatedly to decide — according to their taste and campaign-design needs — how planar areas work and are laid out. In that context, it has only this to say about how demiplanes work:

Demiplanes […] boast their own physical laws. (DMG, p. 68)

The spell demiplane creates a demiplane, and that demiplane that it creates is not an exception to this rule. Beyond size, shape, and wall composition (and the implicit feature of having a non-lethal atmosphere, which I think we can take as read) all other parametres are the DM's bailiwick.

Should this eventuality come up, the DM will have to think about what physical laws the demiplane has, such that it boasts a human(oid)-breathable atmosphere. Perhaps it creates infinite air as needed; perhaps it suspends the need for air; perhaps it has a fixed quantity of air; perhaps it (like a modern house) maintains its breathable air by exchange via the cracks around its outer fixtures (i.e., its door) and becomes a deathtrap once those cracks disappear with the door.

The game is silent on this: consult your DM — or if they're an immersionist DM and won't tell you because your PC wouldn't know, I suggest that a responsible wizard will do due diligence and research this issue before it becomes a matter of life and death.

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About a month at most.

This article explains that the real issue here isn't actually oxygen deprivation, but Carbon Dioxide poisoning. In a 10 foot cube room, he calculates that a person at rest could survive for ~24 hours, half that if they're moving about and actively doing things (ie: looking around for an exit), and half again if they're doing strenuous work (ie: trying to break down the walls).

Extrapolating from there, a 30 foot cube room has 27 times the volume, and thus you should last ~27 days at most, if you do nothing but sit there. Depending on how active you are, that could drop down to as little as 2 weeks. Theoretically it could go even lower, but you would have to really work yourself hard every day and essentially not sleep.

However, there are also other factors to consider:

Water

However, there is also the matter of water. If you brought along a good supply, then great! You'll probably live long enough to suffocate to death. If not, then you'll start running into problems around day 3.

Exactly how long you can survive without water is difficult to predict, but 5 days without water is probably about as much as you should expect, though depending on circumstances and your condition, you may last a few more days after that, but no more than 12 at the very most.

Food

Food is also a problem, but less of one. You can live for about 3 weeks without food, though it'll start causing problems before that, especially if you're being active. You can probably squeeze through that last week before you suffocate, and if you brought any kind of food along, even if it's just leather to chew on, you should have no problem lasting the full month.

Sanity

Of course, before any of this becomes an issue, you'll have your sanity to consider. Being alone in a completely silent room can be pretty devastating. Solitary Confinement is often considered a form of torture, and while I don't have any exact numbers to share, it's definitely a possibility that you will run into this in the timespan given. You could very well survive the full month and be rescued just in time...only for your rescuers to find that you've gone completely insane from the isolation.

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5e PHB on page 183 reads:

Suffocating:

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.

then you require reviewing the involved creature's provisions, to check if they will Starve/Dehydrate to death first, mostly anecdotal, though useful:

5e PHB on page 185 reads:

Food and Water:

Characters who don't eat or drink suffer the effects of exhaustion (see the appendix). Exhaustion caused by lack of food or water can't be removed until the character eats and drinks the full required amount.

Food:

A character needs one pound of food per day and can make food last longer by subsisting on half rations. Eating half a pound of food in a day counts as half a day without food.

A character can go without food for a number of days equal to 3 + his or her Constitution modifier (minimum 1). At the end of each day beyond that limit, a character automatically suffers one level of exhaustion.

A normal day of eating resets the count of days without food to zero.

Water:

A character needs one gallon of water per day, or two gallons per day if the weather is hot. A character who drinks only half that much water must succeed on a DC 15 Constitution saving throw or suffer one level of exhaustion at the end of the day. A character with access to even less water automatically suffers one level of exhaustion at the end of the day.

If the character already has one or more levels of exhaustion, the character takes two levels in either case.

Now that we've got these defaults defined, I will state there is one mitigating variable factor: Available O2 Concentration; yes you should PRIMARILY look into C1-O2 Poisoning being their ultimate suffocation factor, but; 1% C1-O2 in 1 minute of O2 is way less than 1% C1-O2 in 1 month of O2; this begs the question:

How long was this Doorway open to allow breathable O2 in?

Presuming the Room got FILLED with O2, completely: our next reference should look into WHERE this Demiplane ACTUALLY should be located, to find any predefined rules, from what my research has told me the Demiplane Spell should be creating this Demiplane in the Deep Ethereal; in my research I will presume you follow the Great Wheel Cosmology:

Page 48 in the DMG reads:

Ethereal Plane:

...

The Ethereal Plane is a misty, fog-bound dimension. Its "shores," called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. Visibility in the Border Ethereal is limited to 60 feet. The plane's depths comprise a region of swirling mist and fog called the Deep Ethereal, where visibility is limited to 30 feet.

Characters can use the etherealness spell to enter the Border Ethereal. The plane shift spell allows transport to the Border Ethereal or the Deep Ethereal, but unless the intended destination is a specific location or a teleportation circle, the point of arrival could be anywhere on the plane.

Border Ethereal:

From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy. Ethereal denizens watch the plane as though peering through distorted and frosted glass, and can't see anything beyond 30 feet into the other plane. Conversely, the Ethereal Plane is usually invisible to those on the overlapped planes, except with the aid of magic.

Normally, creatures in the Border Ethereal can't attack creatures on the overlapped plane, and vice versa. A traveler on the Ethereal Plane is invisible and utterly silent to someone on the overlapped plane, and solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal. The exceptions are certain magical effects (including anything made of magical force) and living beings. This makes the Ethereal Plane ideal for reconnaissance, spying on opponents, and moving around without being detected. The Ethereal Plane also disobeys the laws of gravity; a creature there can move up and down as easily as walking.

Deep Ethereal:

To reach the Deep Ethereal, one needs a plane shift spell or arrive by means of a gate spell or magical portal.

Visitors to the Deep Ethereal are engulfed by roiling mist. Scattered throughout the plane are curtains of vaporous color, and passing through a curtain leads a traveler to a region of the Border Ethereal connected to a specific Inner Plane, the Material Plane, the Feywild, or the Shadowfell. The color of the curtain indicates the plane whose Border Ethereal the curtain conceals; see the Ethereal Curtains table.

...

Traveling through the Deep Ethereal to journey from one plane to another is unlike physical travel. Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it's impossible to measure speed and hard to track the passage of time. A trip between planes through the Deep Ethereal takes 1d10 × 10 hours, regardless of the origin and destination. In combat, however, creatures are considered to move at their normal speeds.

From this Data we found:

The Ethereal Plane also disobeys the laws of gravity; a creature there can move up and down as easily as walking.

Additionally, this extra information prevents certain results: Exertion to attempt to destroy Walls gets limited from:

Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it's impossible to measure speed and hard to track the passage of time.

Depending on if you follow specifically Forgotten Realms Lore, you would find this information on the Ethereal Plane useful:

...

When travelers crossed into the Border Ethereal, they and all their possessions were converted to their Ethereal equivalents—metal became ethereal metal, flesh became ethereal flesh, and so on[Citation: Jeff Grubb (July 1987). Manual of the Planes 1st edition. (TSR), p. 12. ISBN 0880383992] —allowing free movement (in most cases) in any direction through the solid matter of the adjacent plane [Citation: Jeff Grubb (July 1987). Manual of the Planes 1st edition. (TSR), p. 11. ISBN 0880383992]. Since everything was permeated with ethereality, an air-breathing creature could breathe ethereal air and could not drown in an ethereal lake nor be crushed by an ethereal rock. However, not all in the adjacent plane was insubstantial. Living things larger than one-celled animals generated an aura that radiated around them and prevented passage to their interior, so an ethereal traveler could not place a weapon inside a living creature where it would materialize and cause damage. A jungle would be an extremely torturous maze to navigate; it would be much easier to float above the vegetation or pass below its roots. Dense metals such as lead or gold also prevented passage of ethereal matter. And finally, some magic spells and alchemical mixtures could form an effective barrier [Citation: Jeff Grubb (July 1987). Manual of the Planes 1st edition. (TSR), p. 12. ISBN 0880383992].

...

When you passed through a curtain into the Deep Ethereal, time slowed down to one tenth the rate it flowed in the Border Ethereal and the plane that it bordered. For every ten hours spent in the Deep Ethereal only one hour passed on the other side of the curtain. Metabolic and other natural processes slowed down also, so it did not feel like ten hours, when you crossed back through a curtain into a Border Ethereal you were only an hour older and an hour hungrier [Citation: Jeff Grubb (July 1987). Manual of the Planes 1st edition. (TSR), p. 12. ISBN 0880383992].

... [Citation: https://forgottenrealms.fandom.com/wiki/Ethereal_plane in case you want to look into this version more]

This GREATLY extends survival time, and would solve our Breathable Atmosphere query, presuming you are following the Great Wheel Cosmology Model.

NOW we can calculate a more exact number.

If you follow Forgotten Realms' Great Cosmology Model you would find the most patient inhabitants survive a meager 64~65 Hours, around two-and-one-half days, though the creature would feel as though they spent around 648 hours trapped in the room (~27 days).

The reason this discrepancy sidestepped their 10:1 scale unsurprisingly stems from exertion; their character ages slower, though their body will continue requiring O2 when they exert themselves, primarily from interacting with objects (a Demiplane Home would be one example), you could exclude exertion from running: this gets handled with willpower (Intelligence, Mental Fortitude), very useful to powerful Wizards.

Beyond this point; you enter World Building territory.

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  • \$\begingroup\$ What supports the assumption that the demiplane is in the deep ethereal? The DMG categorizes them in the Other Planes section which states that these planse are "A variety of realms exist between or beyond the other planes". \$\endgroup\$
    – GcL
    May 20 at 21:13
  • \$\begingroup\$ @GcL You just described the entire Ethereal Plane where these Demiplanes got formed. \$\endgroup\$ May 21 at 3:55
  • \$\begingroup\$ The Ethereal Plane and Other Planes are distinct and separate sections in the 5e DMG. Is this answer assuming that the Other Planes is a part of the Ethereal? If so, what text is supporting that assertion? \$\endgroup\$
    – GcL
    May 21 at 6:26
  • \$\begingroup\$ "I will presume you follow the Great Wheel Cosmology" forgottenrealms.fandom.com/wiki/Demiplane "In the Great Wheel cosmology, demiplanes were found in the Ethereal plane because they were formed from the swirling proto-matter that made up the Ethereal.[1] The Demiplane of Shadow was the best known example." "Beyond this point; you enter World Building territory." \$\endgroup\$ May 21 at 17:27
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    \$\begingroup\$ In that case, this answer would be improved by stating up front that it is mixing AD&D and 5e lore. Would also point out explicitly where this contradicts the DMG. \$\endgroup\$
    – GcL
    May 21 at 18:56

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