At 20 000 ft you've got approx 25mins or less before hypoxia symptoms become apparent.
I beg to differ - I've actually done it, and no no physical exertion I was rapidly heading towards being a blathering idiot in a mere ten minutes. (Hard to tell from the normal state though, I must admit!
)
I guess that's a pretty average normal figure in the real world.
I did it in an Aerostar 601P endorsement, for the express purpose of knowing what hypoxia feels like. I've never forgotten it, so I'm always aware.