One pilot on 100% oxygen is going to have far greater supply than the supplemental pax system will provide.
About the 10th time the subject has been raised, but still it is not confirmed what type of emergency air the passengers had on this flight, bottled or chemical.
Also not confirmed how many portable bottles of air were available to cabin crew, (8 has been suggested)
Without answers to these questions it is not possible to say that the pilot supply would outlast one or two savvy cabin crew using portable supplies and (if chemical air was available) activating any unused chemical masks in loos or in areas where no passengers were seated. There were at least 50 empty seats although one chemical generator is shared between a few seats. (170 generators suggested)
However apparently not everyone can survive for extended periods at 35,000ft without a pressure mask.
So like all the scenarios and sub plots of this event, it is possible yet unlikely.
More detail in previous posts..