Rapid Decompression
AC 61-107A from the FAA has the most authoritative data I can find.
The table below indicates the time of useful consciousness from a rapid decompression at 40,000 ft is 7 to 10 seconds. If the pilots chose to initiate the descent before they donned their oxygen masks, it may have been too late.
TABLE 1-1. TIMES OF USEFUL CONSCIOUSNESS AT VARIOUS ALTITUDES
........................Standard Ascent Rate........After Rapid Decompression
Altitude (Feet).............Time..............................Time
18,000.................20 to 30 minutes............10 to 15 minutes
22,000.................10 minutes.....................5 minutes
25,000..................3 to 5 minutes...............1.5 to 3.5 minutes
28,000..................2.5 to 3 minutes............1.25 to 1.5 minutes
30,000..................1 to 2 minutes...............30 to 60 seconds
35,000..................30 to 60 seconds............15 to 30 seconds
40,000..................15 to 20 seconds............7 to 10 seconds
43,000..................9 to 12 seconds..............5 seconds