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:
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 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.
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.