here in Chile we have the same regulation, above FL350 if one pilot leaves the station the other one has to wear the mask and breathe Pure magic O2.
I can speak for myself and the Captains I fly with, we follow the procedure, you never know what can happen.. and like you said, we 90% of the time are above FL350...
they could be really close to you (thats why called Quick donning) but in a loss of presurization that high, don't have many time to think about it..
and flying the last leg of the day at 6-7am, a little bit of pure O2 is not bad for wake up
