Altitude - Moderate Activity - Sitting Quietly
25 000 feet - 2 minutes - 3 minutes
28 000 feet - 1 minute - 1.5 minutes
30 000 feet - 45 seconds - 1.25 minutes
35 000 feet - 30 seconds - 45 seconds
40 000 feet - 18 seconds - 30 seconds
45,000 feet - ?? seconds - ?? seconds
None of this makes any sense from the training I received
Text book figures have a lot of latitude, as they cater for the very average person trying to do complex calculations. If you are super-fit, used to high altitude, and merely sleeping, you can go much higher. I spent three days at 21,000 ft, with no oxy at all, and doing some hard hiking (although I had to stop every 100m).
The real problem is temperature. The wheels might keep you warm for a while, and the warm fuel likewise for a while, but I imagine that the center section gets quite cold after a few hours.