Common occurrence in the AZ heat. Go to the store, come back out in 5 minutes and battery is dead. I've had them die in 1 month; most do not make it through the warranty period. However, the Japanese batteries that came in a couple of our Toyotas lived beyond the warranty.
I've had a couple batteries from WalMart that lived almost 5 years, which is better than most batteries do in this climate.