With a 45ma drain, your battery should last for several MONTHS. If you want to get an exact number, look up Peukert's law and do the calculation yourself. A group 49 battery has a capacity of about 100AH@20A, and K=1.15.

You should have your battery load tested at any parts store, it can be weak even though it can be charged to 12.7v. The problem may also be caused by an intermittent draw from something like an alarm system or electric vacuum pump.