The time of useful consciousness at 35,000 feet has been found to be 30 to 60 seconds. Rapid decompression at 35,000 feet MSL could cut that time in half, due to forced exhalation of air from the lungs, but 15 to 30 seconds is still reasonable time to don masks.
Source: http://en.wikipedia.org/wiki/Time_of_useful_consciousness
Didn’t work for Payne Stewart.