Sounds like a job for AppleScript/Automator -- or perhaps a third-party screen recording program.
I'll have to poke around a bit when I get a chance. QuickTime has a start and stop command for recordings listed in the Automator actions. The recording could p0roblably be timed with Automator's loop function or by setting two different iCal alarms. The first alarm would contain the Automator command to start a recording while the second, set an appropriate number of minutes later, would contain the stop command.
I tried it a minute ago with the loop command but something didn't work.