I used nitrous oxide just one time, and it wasn't to get high: this was before the web, and I had read somewhere that the feeling of the need to breathe is not a result of lack of oxygen, but a buildup of carbon dioxide. My friends were doing whippits, and I figured that nitrous oxide wouldn't allow for the production of carbon dioxide, so if I breathed it -- no sense of need to breathe, right?

So I breathed out as much as I could, breathed in a full lungs-full of nitrous, and held my breath. I wish I had timed it, because I have no idea how long I held my breath, but at least 15-30 seconds, because I held my breath for a bit, then I was talking a bit (not breathing in, just out) somewhat, then holding my breath again and thinking.

My whole body went numb. I was giddy. My vision closed in and started to dim. And through it all I felt no need to breathe at all. As my vision was going away I was idly thinking, "Hmm, still no need to breathe. But my vision is going away, so I should probably breathe. What was I thinking? Oh yeah, no need to breathe but I should breathe anyway. What was that? Oh yeah, breathe before I lose consciousness because these idiots are all high and if I just died here, they'd let me. What? Oh, breathe, moron." And I breathed.

Like I said, I didn't time it, so I don't know how long it lasted. I idly breath-hold for fun, and I can easily go two minutes with no prep, but obviously that's with oxygen in my lungs, so it could have been anywhere from 20 seconds up to a minute or more?

I did the counter-experiment a bit later: we had dry ice, so I put some in the sink and let it build up to a foggy unspecified concentration. I breathed all the way out, carefully lowered my face in, and breathed in.

Or, I tried to. My entire body was instantly on fire, and I violently coughed it out in a split second. Hypothesis confirmed! :-)