[RPG] How long would it take to suffocate in a demiplane created by the spell of the same name


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.

Best Answer

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.