What's the longest a car can sit without running before its' battery needs a jump?

[SilverBack]

Tried searching this first but got nowhere, so here goes:

I just replaced the car battery in my landlady's Accord after she couldn't start it. This was due to her being out of town for 3 months straight. AAA guy was afraid to jump it because his tester showed it was sitting around long enough to explode if it was jumped.

Anyways she's going out of town again next week and I convinced her to leave her keys with me and another roomie so we can take turns firing-up the engine every so often to keep the battery alive. My initial thought was this should be done once a week, but I can't help to think it could go at least an extra couple weeks longer without running.

I can try to make the time to run the engine for a few minutes every other week, but I won't always have time to switch-out the battery just because it sat around for too long, so any thoughts and experiences would be a great help!