Assuming a 2000/3000 fpm descent, from 35k ft the aircraft would descend to an altitude that is capable of sustaining life in under 5 minutes, and in under 10 minutes to a level where full consciousness can be regained in a matter of minutes. However if there is any extended time spent above 8000m (that is more than max 5-10 minutes), then the answer is probably never.
Thanks for that, although why my original post positing a hypoxia theory has vanished, I don't know.
Unlike the Helios tragedy where the aircraft kept flying in the 'death zone', flight MH370 started descending so at some point pasengers and crew would have begun to regain consciousness. Would the aircraft then have been too far from the mainland to make cell contact?