At 35,000 feet you’re dead in seconds without an oxygen tank.
When I was a C-5 flight engineer, a visit to the altitude chamber was required. They asked for two volunteers, one to go to 30,000 feet and the other to 35,000 feet. The rest of the group went to 25,000 during that phase. I volunteered for 35,000. At 35,000, I took off my mask and started to place different shapes into a child clear plastic ball. For the first 25 seconds, I was doing well and then I felt myself slowing down. Then suddenly I felt that I had a whole bunch of energy and was putting more shapes into the ball. The instructor asked me to put my mask on and gang load my regulator. As I looked for my mask, I realized that the instructor had already secured it to my face and that was the energy burst I experienced. My time of consciousness was about 30 seconds.