I wonder how long a person can stay alive at 25,000 ft with no additional oxygen?
Minutes? Seconds? It certainly isn't hours.
It was suggested that depressurization was slow, as in falling asleep. Radio transmissions say another plane flew in to see oxygen masks rising and falling. What happened after that is unknown.
Breathing but unconscious and slumped over.
Years ago, in the military, we were in a pressure chamber at a simulated 25000 feet. I took off my oxygen mask and found I thought was just fine till the Sgt told me to put the mask back on.
I felt OK, but everyone in the chamber said I turned a beet red from lack of oxygen. I’m going to say, you can not last more than a minute or two.
Also, when depressurization comes, the cabin fills with fog.
Many days, with proper acclimitization, though it's pretty dangerous. Everest has been climbed many times without auxiliary oxygen, and it's just under 30,000 feet.
Somebody who's acclimated to sea level would have a really rough time, though.