I'm pretty sure that military pilots have to go unpressurized to 25,000 feet in order to be qualified. however they don't have to stay there for very long. and certainly not as long as the duration of a commercial flight.
We basically had to do menial tasks, like read a deck of cards, play "paddy-cake", etc., and then put our oxygen masks back on once we thought we were too impaired, while a technician was there with us.
However, I think the simulated altitude was 20k or 25k feet.
Needless to say there's a reason that O2 is (legally) required in unpressurized aircraft for anything over 10k feet.
I have flown at 12k for 1 hour and could tell that the air was much thinner and that going much higher would have started to impair.
I think that most commercial aircraft run somewhere between a 6k and 8k cabin altitude and as such a whole different set of rules apply. I don't know for sure - I'm only a private pilot.