The cert requirement is for all passengers and crew for 15 minutes - or - for as long as it takes to get from max certified cruise altitude to 10k ft., whichever is longer.
O2 'on demand' is a potential fire hazard (and aircraft fires have killed far more than have died from inflating life vests inside the aircraft).
At least in this part of the world, instructions on use of life vests during the pre-flight passenger briefing include the instruction to not inflate until you're out of the aircraft. People ignoring the pre-flight safety briefing falls into Darwin candidate territory.