From my Emergency Training Manual.
The "Time of Useful Conciousness" at any altitude is the maximum length of time the exposed individual has to perform the useful tasks necessary for his survival (such as donning an oxygen mask) after a rapid decompression, is in the table below.
25000ft 2mins
30000ft 45secs
35000ft 30secs
40000ft+ 18secs
The time of 18secs at 40000ft and above, does not change, because 18 seconds is the shortest time it takes for oxygen starved blood from the lungs to reach the brain and depress brain function.
Recovery, after administrating oxygen, will require about the same length of time.
Also goes on to say -
The degree of hypoxia varies according to the amount of oxygen deficiency, and its onset depends on the rate of decompression.
The danger of hypoxia is that not only does the subject not notice the gradual deterioration, but also he may actually experience a feeling of well being.
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rgds Rat