Battery life can depend on a lot of things, including weather and how often it gets a charge. Use the above post to check the voltage outputs, pretty easy to do with a multi meter across the battery terminals in about 5 minutes, ie check that the voltage does in fact increase with revs and then level off and not go over the prescribed limit, from memory around 15volts.

From your description I think it's likely to be OK and is just use related. If so you can buy a battery tender, which allows easy trickle charging of you battery and maybe do that once a week. Alternatively the light weight battery technology has come a long way and IMHO are now worth looking into. If you go down this track don't buy cheap, as some LiPo batteries are dangerous, someone like Speedcell uses non-volatile cells, so whilst more expensive, they won't melt your butt.