Pete gave good ideas. I'd start with checking alternator charging...if that's low, it will slowly kill a battery. Grounds can definitely be a culprit. If it's charging well, test the battery load with the car off. If it is much of anything, it could drain the battery over a short time. If you have a load issue, check to make sure it isn't something like one of the courtesy lights that the switch has broken on and so it's on all the time. It could be something simple like that.

This definitely is not normal. I had a 5 year AC Delco battery last exactly 5 years in Ontario winters, which included some vicious abuse with rally lights and such. It was parked outside the whole time There's an issue somewhere in your charging system or you have some small drain.

Good luck!

Nate Gundy
--
1997 850, 1979 242GT, 1990 740GL