Being a retired military pilot, I lost track of how many alt chamber rides I've done. If I had to guess, I'd say between 8 and 12. The one thing everyone needs to understand, is that everyone is different in their ability to function at altitude. For me at 25k, I've got about a minute or less to recognize hypoxia, and get on backup O2. Probably less now that I'm older. I remember one guy from Colorado Springs (6200ft) that went almost 5 minutes. The safety guy in the chamber finally told him that was long enough, and to put his mask back on. He was able to comply!!! I'm sure there are Sherpas from Kathmandu that could go a very long time at 25k. There were other pilots in the chamber, that became euphoric, requiring the safety guy to put their masks back on for them.
The most important thing is to recognize hypoxia, and get on backup O2 first. Do not waste time trying to trouble shoot your malfunctioning system, or start your descent. If I had a one button emergency descent system as part of my autopilot, I'd probably push it as I was reaching for my bottle.
You must have a backup bottle, that's fully charged, within arms reach, that you function tested prior to your flight.
Once safely on your backup O2, start your descent and declare an emergency!
It behoves everyone flying in the flight levels, to do at least one alt chamber ride. You'll never know how you're going to react till you do.
Bob
Everything above is my opinion.