A bit after i did full trimix (T2) a couple of guys and me experimented breathing hypoxic mixes in a friendly filling station... while breathing 15/55 i could do push ups, jog in place for a couple of minutes before feeling anything, even with real hypoxic mixes, without exercise it takes a bit... bit the kicker was pure He. Everyone of us would pass out after 2,3,4 breaths.. you dont feel anything and blackout comes up so fast you dont have time even to take the reg out of your mouth. As if flipping a switch. Gaining consciousness once you breath air again is almost as fast.