To answer Pontius several pages back
However I don't know whether the drop downs work on a common system, ie one group are exhausted even is some drop downs have not been used. If that is the case then those crew on drop downs will be no better off than the pax.
It would depend on the system fitted, if chemical generators than only the masks in the units where one mask was pulled would flow. Therefore any units above unoccupied seats which were not activated would be available to the crew. I know on the 777 I trained on each unit lasts approximately 22mins. We know there were c50 empty seats on this flight...
However if it is a gaseous (bottled) systems as per 747, which one poster has suggested it was, duration would vary depending on altitude, number of masks in use and how many bottles are fitted to the system. Theoretically if only a couple of CC were quick enough they could long outlast the flight deck oxygen which is only designed for 2-4 people to use for as long as it takes to make an emergency descent. Of course if the pax are on it, it wouldn't last as long with 250+ using it...