The answer to this Fermi problem is a bit surprising.
Assuming you're not in a big lecture hall and the professor shuts the door at the start of class, how long does it take for you and your classmates to deplete the oxygen enough to feel it?
Here's a taste of the reasoning behind the answer:
So one person needs about 2lb of oxygen a day, or .9 kg. But how many liters is that? Oxygen has a molar mass of 16 grams, so oxygen gas, or O2, has a mass of 32 grams per mole. One mole of gas at standard pressure and temperature takes up 22.4 liters.
A commenter over on Fine Structure notes that CO2 is more of a problem than oxygen.
I don't know if they brought this up on physicsbuzz yet, but lack of oxygen isn't really uncomfortable (though it can kill you). Increase in CO2 is what triggers the apparent need to breath. I am pretty sure the minimum partial pressure of O2 is around 0.16 bar. Actually, that is the min recommended, I don't know if that is the pass-out limit.