Would failure to put on an oxygen mask during loss of cabin pressure result in death?
I did high altitude training about three months ago, up to 30,000 feet simulated altitude.  Here's the timeline:

Within 30 seconds (oximeter 100-95): noticed physical symptoms beginning.  I could tell that something was wrong, but felt functional.

Within 60 seconds (oximeter ~90): mild headache.  Still able to write my name.  Not feeling good, but acceptable.

Within 3 minutes (oximeter ~80):  Able to do simple tasks when specifically directed (thread a nut onto a bolt), but completely unable to multitask at even the simplest things (tried to put my pen down...didn't notice that I missed the desk - it landed on the floor.)

Within ?? minutes (oximeter ~70): I am no longer able to perceive the passage of time.  No longer able to take meaningful notes.  My signature is still legible - instinctive muscle memory apparently doesn't degrade nearly as fast.

Within 6 minutes (oximeter unknown): I have taken another pen from my pocket and am playing the knife game from &quot;Alien&quot; with it.  This is much more interesting than paying attention to the directions from the instructors.  I don't notice anything odd about my behavior.

Immediately after:  An instructor gives me a direct command to put my mask on.  I comply.


Total time in chamber: 7 minutes.  Time of useful consciousness in chamber: about 2 minutes

So, to answer your question: It depends.  I can safely say that seven minutes at 30,000 foot environment won't guarantee death (because I've survived it), and it won't guarantee severe brain injury (they let me keep flying) but on the other hand, I'm young and I exercise.  The best answer I can give is the same one that applies to scud running:
Just because the guy before you was able to do it, doesn't mean you'll be able to do it.