Its not that simple.
Whilst clearly at 30,000 feet you have a very short time until everyone is unconscious (a Beechcraft KingAir was lost here recently because it depressurised and then flew 1000 km further on autopilot with no-one at the controls till it eventually ran out of fuel) there is no "magic" altitude and it effects differently people differently.
The higher you go the shorter the time you have before apoxia sets in. At 10,000 feet it can take as long as 10 minutes before effects are noticeable.
Pilots acclimatised to high altitudes can handle it better. Also some people function with oxygen deprivation better than others.
This website gives an OK introduction to the topic.
http://www.allstar.fiu.edu/aero/oxygen.htm