"Hmmm... how much of that time would be used just to realize that SOMETHING is happening, come to terms with the situation and decide that the right action is to go get an oxygen mask?"

About as much time as it takes to realize that slamming on the brakes is a better option than slamming the accelerator to the floor when you see that speeding semi running the red light at that intersection.

As Lono said, if an oxygen masks drops in your lap, put it on.

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"I think the maneuver is to roll the plane on its side and unload the wings into something like a sideways 0 g condition. This drops the plane quickly and doesn't build up airspeed like a dive would."

I think that's called a side-slip.

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"Pretty hypothetical, but would it be possible to share air, as one does in scuba emergencies, if you could not reach your own air supply?"

Do the words 'From my cold, dead hands!' mean anything to you?

Sue