The paxs oxygen drop out masks aren`t checked daily. It would take for ever to restow every mask.
The length of time the oxygen lasts for depends on a/c type and what type of system they use. The newer a/c use a chemical generator which last between 12-15 mins on short haul aircraft, while EOPS 757 lasts 22 mins. There is one generator located above each row of seats each side with either 3 or 4 masks attached, if 3+3 seating. Pulling one mask in that row activates the flow to all the masks in that row ie the A B C or D E F side.
So it does not matter how many paxs are on board the a/c.
Older aircraft (DC9) for example had large tank in the hold.
The system works in a number of ways but on the whole they drop automatically now days with the drop in cabin pressure(except early 146??). When they do drop its very noisy with all the compartment doors opening. The pilots also have a manual backup to make sure the masks drop which they will activate. Also once the aircraft has levelled out and the command by the Capt/FO says it OK the cabin crew will carry out checks and also drop any masks from the PSU as neccessary.
The most usual decompression is a slow one, where the masks will drop and paxs don`t realise anything is wrong...they can still breathe so they just look at the mask and carry on with there in flight meal, drink or reading The Times.