The term that you would want to research is "Time of Useful
Consciousness (TUC)"... If it is a gradual pressure loss, you might not
realize it and basically just drift off to sleep...

Tlme of useful
consciousness
Altitude (ft) without oxygen

40,000 15 seconds
35.000 20 seconds
30,000 30 seconds
28,000 1 minute
26,000 2 minutes
24,000 3 minutes
22,000 6 minutes
20,000 10 minutes
15.000 Indefinite

Source: "Physiologically Tolerable Decompression Profiles for
Supersonic Transport Type Certification," Office of Aviation Medicine
Report AM' 70-12, S. R. Mohler, M.D., Washington, D.C.; Federal
Aviation Administration, July 1970.