A question of height, pressure and time. IIRC, for flight over 40k for flight up to 50k you could use a mask alone with a Mk17 regulator delivering an overexposure of 30mms O2.
With a Mk21 you were limited to 45k on the mask alone.
If you wore a pressure jerkin you could again go to 50k but also received a greater partial pressure, 45mms I think.
If you added G- pants you could go to 56k and 70mm O2 over pressure. Beyond that it was a function of your neck, gullet, ears and tear ducts to hold a greater pressure. Therefore for flights to 66k a Taylor Partial Pressure helmet was worn. I don't know its pressure limit.
In the event of loss of pressure a rapid descent had to be initiated within 30 seconds and a rate of descent of at least 10k/minute to 40k.
In the event of a bailout your ROD would be much lower. I believe a Canberra crew was rejected by their aircraft at very high altitude, probably in 1960s and were reputed to have white hair when they eventually landed. Rumour had it that a photo flash ignited under the aircraft.