Until it runs out ?
A lot longer than it takes you to descend to a breathable altitude.
At 20 000 ft you've got approx 25mins or less before hypoxia symptoms become apparent. ( loss of muscular control, judgement,reasoning, memory and time sense, emotional outbursts)
At 40 000 ft you've got approx 15-20 secs (10 secs under rapid decompression ), until loss of consciousness.
Also smokers will encounter symptoms 2000ft to 7000ft earlier than non-smokers
regards