5e PHB on page 183 reads:
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.
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.
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. (...)
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.