As a pilot in the US, I'm required to wear oxygen at altitudes that are lower than I've hiked, however the degree of mental challenge involved in hiking is just a bit different than that posed by flying! Also, physiological consequences of low O2 saturation tend to manifest in ways that a pilot will find difficult to recognize since many of the symptoms actually mask themselves. For this reason many pilots opt to wear a pulse oximeter to monitor O2 sat on the fly, so to speak.
Since you probably don't really care about the legal aspects of oxygen use by aircrews (but if you do take a look here:
http://www.risingup.com/fars/info/part91-211-FAR.shtml) take a look at the data for "time of useful consciousness" which gives a pretty good generic answer to your question. Technically most people will have many symptoms of hypoxia much sooner than these charts list, but these charts are aimed at how long a pilot has sufficient mental ability to continue to perform required duties - not the time before any impairment at all.
[edit] Here's one such table:
http://en.wikipedia.org/wiki/Time_of..._Consciousness