Actually that battery only needs a 10-11hr charge time, assuming 500mA charge rate, going by the on-line charge time calculators.

If you've been charging it for longer intervals you may have prematurely ruined the battery.

Also how quickly do you normally discharge it?

I have a small Hobbico electric starter pack on one of my e-starters, and I find that the battery only lasts a season, because of very heavy use.