I personally experienced breathing nitrogen until loss of consciousness under controlled and supervised conditions for training purposes with the RCAF. I was in a room with seven other people who were all doing the same thing as well as instructors who were in here with us for safety.

The point of the exercise was to sit in a room with a mask on, recognize the symptoms of hypoxia when we experienced them and throw a lever that would resume normal air breathing once we had enough. We were given tablets with simple games to play to simulate having our minds occupied on accomplishing some tasks. We knew they were going to switch or air supplies with pure nitrogen at some point to cause hypoxia but we didn't know when it was going to happen. The room was also a hypobaric chamber but it didn't stimulate a high enough altitude to induce hypoxia by itself, it was only there to simulate the environmental signs of decompression ( fogging of the air, percieved drop in pressure, cooling sensation, etc)

We sat there for a few minutes accomplishing the tasks on the tablets (basically paying candy crush) with nothing special going on. Then I noticed that we all started breathing deeper and harder. When I looked around people were also red in the face but strangely did not feel any discomfort from it and some people were even still playing on their tablets without noticing. Some of them threw their personal lever immediately because the point of the exercise was to recognize the signs of hypoxia. But others including my competitive ass wanted to see how far I could take it and if I could outlast others so we kept going.

My breathing naturally got deeper and harder but strangely I wasn't feeling like I was suffocating. I started feeling pins and needles in my extremities. Concentrating on the tasks in the tablet became increasingly difficult and slower. A few moments later I got tunnel vision and my hearing started to sound muffled. These two effects progressively got worse until I could almost not see or hear anything anymore at which point I finally threw the lever just before passing out due to a phenomenon called oxygen paradox where when oxygen supply is resumed the hypoxia symptoms briefly get worse before going away. I didn't even notice passing out. I woke up a few moments later and from my perspective it seemed that time had skipped forward a minute. Had I not thrown the lever and there were no instructors to do it for me I would have died a few moments later.

All of this took less than 5 minutes and I never experienced anything worse than mild discomfort throughout. I don't know how they managed to make it last 25 minutes other than maybe the brain stem running on fumes and keeping the heart beating but there is no consciousness at that point. If I ever had to pick a way to be executed this would be it, provided that it is done correctly.