I'm an aviation student at UND and am enrolled in a flight physiology class. We just finished going in the altitude chamber with a decompression at 25,000'. Like that last poster you begin to notice effects at about 3 minutes, of course this varies with each individual. The person next to me passed out in about 2 min 45 seconds. At 21,000' it is about 8- 10 min, at 25 it is down to 3 - 5 and at 30 you have about 1 - 2 min.