My car is dead. Why?

I have a 2011 Mazda3 sport, and it's the fourth time the battery dies for no apparent reason. First 3 times (all in a one week span), the car would start after a boost, but the battery would not keep its charge, even though I had it running for more than an hour straight. So they replaced the battery with a new one. This morning, barely two months later, the same thing happened. They can't find a reason why this would happened, other than I hadn't used the car in two days. Is it really long enough for the battery to go dead like that? I mostly use my car during the weekends, and it's usually for long drives (2 hours at a time). What can cause this? My driving habits have not change, and I never had this problem with my Mazda 2008.