The higher you go above 10000ft, the shorter your legal time of useful consciousness. It depends a lot on individual fitness and age as well of course. Also, aclimatisation helps - hence athletes doing altitude training. The blood thickens in response to thin air in order to carry more oxygen. Typically, most people are OK for about half an hour at 14000ft but once you get up to 30000ft ish you are down to 12 seconds which is the time it takes the blood to get from the lungs to the brain.
The highest altitude for sustained life was demonstrated by some Andean silver miners. They lived at 17000 ft, I believe, and walked up to the mine entrance at 18000 ft each day before descending to the workface at 17000 ft inside the mountain. The mining company moved their village up to 18000 ft to save the time taken to walk to work. This was OK for the miners who went down to 17000 ft each day, but their families complained of persistent headaches because they were at 18000 ft all the time.