"Most FDRs record approximately 17–25 hours worth of data in a continuous loop."
http://en.wikipedia.org/wiki/Flight_data_recorder#Design
EDIT: ah, it looks like you are talking about the cockpit voice recorder, which records 2 hours of data: http://en.wikipedia.org/wiki/Cockpit_voice_recorder
Yeah - sorry I should have clarified voice recorder not the flight data recorder. Will edit.
"Most FDRs record approximately 17–25 hours worth of data in a continuous loop."
http://en.wikipedia.org/wiki/Flight_data_recorder#Design
EDIT: ah, it looks like you are talking about the cockpit voice recorder, which records 2 hours of data: http://en.wikipedia.org/wiki/Cockpit_voice_recorder