This is a table that shows Time of Useful Consciousness at a couple of altitudes. It shows the time that you may expect an average human in reasonable health to be able to function with rational thought.
FL 350 30 to 60 sec 10,668 m 35,000
FL 400 15 to 20 sec 12,192 m 40,000
A rapid decompression can reduce the TUC by up to 50 percent caused by the forced exhalation of the lungs during decompression and the extremely rapid rate of ascent. These times will be modified by factors such as alcohol intake, recent exertion, the ability of the blood to transport oxygen etc.
The brief has to take into account the worst case, so for instance a 777 flying at 40000 suffering an explosive decompression. This may only give you 7-8 secs in which the mask must fall, you must recognise the problem - startle factor is a significant issue as is disbelief, and to then put your mask on. If you stop to do the child first, you may both die, if you sort yourself out first the child may suffer a few moments of unconsciousness but nothing serious.