The time would be dependant on the quantity of information recorded. QAR's are not necessarily set up to record continuous data, but are switched on by a manual or software command during critical flight phases.
There are a number of different types. The earlier ones have large magnetic tape metal cassettes (which we replace after every flight). The Optical or OQARs have CD type discs in square plastic cases (we usually replace these after 2~3 days, but usually last much longer). Then there are the PCMCIA(?) type (not sure how long these last)... and there are QARs which just send data via small antennae mounted on the front of the unit (just don't ask me how the signal gets through the shielding of the fuselage :P).
Regards.
NSEU