This is a huge debate that's been going on for a longe time. How deep on air is really up to you and your training.

There's the ridiculous 100m club in egypt that do it on air with a bounce dive. And then GUE say after 40m you should be on trimix.

I think everyone is different, i've dove to 80m on air before, it was a difficult dive and a working dive too but sometimes it needs to be done. I would never recommend anyone else do that but sometimes dive theory about partial pressure is exactly that.. theory.

I would say for most, in my honest opinion, 55m is the max, any more then that and you get penalized in a huge way for decompression.