Ah, that brings back memories. That was the exact stuff we did, except that my exercises was to count backwards from 1000 by 13's.
We were told that we could possibly make it 3 to 4 minutes at FL250 before having to go back onto oxygen. I managed to get to 5 minutes, but was was pretty much useless after the 3rd minute. I started to feel a real out of body experience, and it was like I was watching my own body do stuff but I couldn't really control it. I managed to get my own mask on and gangload the 3 switches, but I remember watching my colleagues in the chamber struggle and they had to be assisted by the RAAF team. The guys outside the chamber kept a hawk eye watch on all of us, and the guy in the chamber were quick to assist those of us who were in trouble.
What amazed me was as soon as we 'gangloaded' the switches to go back on full O2 with the masks on, recovery to full lucidity was super quick - within 1 or 2 seconds. It wasn't gradual, it was as if someone just whipped a heavy blanket off your brain!
Side story - before we went into the chamber, we had to be fitted for our helmets and masks, and I was seated in the 'fitting chair' in the crew room as 3 RAAF specialists slapped a bone dome and mask on me. Unbeknownst to them, that mask had just come back from a test facility, and thus it still had the wax seal on the end of the O2 hose to the mask.
Normally, there is no seal, so when the mask is clipped on during a test, you can still breathe normally as the valve at the open end just opens and shuts with your lung pressure. With the wax seal on it though, NO air gets through.
It was a few seconds of odd feeling when they snapped the mask on and I tried to breathe in but nothing happened. I remember feeling really freaked out at the sensation of wanting to breathe, but feeling and experiencing absolutely nothing. I started to grapple with the mask release, but they thought I was just uncomfortable with the fit and they kept slapping my hands away and adjusting the side straps.
I went into panic mode, and I am not sure what prompted me, but I started reeling in the trailing hose from my mask and lifted it up to see the seal on the end. One of the crew saw it and exclaimed before grabbing the hose and ripping the seal off. I gulped down lungfuls of air. Not the best start to my first chamber session!