How long would it take to suffocate in a demiplane created by the Demiplane spell?

Question

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?)

Answer 1

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.

Answer 2

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.

Answer 3

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.

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 CO₂ Poisoning being their ultimate suffocation factor, but; 1% CO₂ in 1 minute of O₂ is way less than 1% CO₂ in 1 month of O₂ this begs the question: How long was this Doorway open to allow breathable O₂ in?

Presuming the Room was 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:

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. (...)

Deep Ethereal:

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.