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.