Physics Buzz presented their Friday Fermi Problem, well, last Friday:

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?

They have presented the answer, and there is some additional commentary at Fine Structure about the problem of CO2 buildup (from Rhett of Dot Physics; which just supports my suspicion that physics bloggers are the main readers of physics blogs)