90 seconds is probably a bit exaggerated but if you are running min loop volume and are relatively shallow it won't take terribly long. For easy numbers let's assume a tidal volume of 1L plus about 3L of "dead space" inside the unit for the loop hoses, canister, and head. 4l of volume at 4ata=16L at surface. You shouldn't run a 1.3 ppO2, but whatever, 1.3/4=32.5%. 16*.325=5.2L of O2 inside of the loop at 1.3. 0.2/4=0.05L *You can't actually do this without adding Dil because you'll collapse the lungs which is why min loop is so important, but we'll roll with it*, so you consumed 5.2L-0.05L=4.7L of O2 consumed. Assuming the "rule of thumb" of 1lpm, that is just under 5 minutes to go from 1.3 to 0.2 at 100ft. If you were at 30ft, it would be right around 2 minutes, and about a minute at the surface. Obviously the opposite as you go deeper since there is more mass of O2 in the loop. More if you don't run at min loop, more if you have a dil that is matched to your ideal bottom ppO2, etc etc. but I'm not sure that I would consider 5 minutes "WAAAAAAAAAAAAAAY" more time than 90 seconds.