DiveGolfSki:Under your scenario, why would the pilots NOT descend to a lower level EVEN if no landing area is possible? If a rapid decompression occurs, you're assuming that the Pilots would insist to fly at altitude and all passengers keep their O2 masks on? Kinda risky.
Perhaps, bigjet would comment on whether or not pilots do or don't descend after a rapid decompression.
The passenger oxygen in modern jets is supplied by chemical production units, otherwise known as "oxygen candles". Once activated, they have a useful production time of thirty (30) minutes.
We have a lot longer, given the size of our oxygen supply. We will NOT remain at altitude for any length of time, however. We will begin descent procedures immediately, because we want you and yours to be OK, and not resemble human popsicles!

The ETOPS plans all call for the aircraft to descend to 10,000 ft first, and then fly to the alternate destination, so fuel planning is based on that.
At altitude, if one pilot leaves the cockpit to go to the head, the other pilot puts on the mask. If there are two pilots in the seat, it is not required. We have quick donning masks next to our seats.