"John Gaquin" wrote
Truth is, if you're cruising at FL350 or higher you've only got about 5
seconds max to get the mask on, but it only takes about one to two
seconds to don the mask and get full O2 flowing.

From the following web site:

http://www.sff.net/people/Geoffrey.Landis/vacuum.html

A larger body of information about how long you would remain conscious comes
from aviation medicine. Aviation medicine defines the "time of useful
consciousness", that is, how long after a decompression incident pilots will
be awake and be sufficiently aware to take active measures to save their
lives. Above 40,000 feet (12 km), the time of useful consciousness is 12 to
25 seconds. (The shorter figure is for a person actively moving; the longer
figure is for a person sitting quietly.)

The number that we were always taught in the airline business was that at
39,000', the time of useful conciousness was 18 seconds.

Bob Moore