SLA (sealed lead-acid) batteries should be recharged every six months. This avoids sulfation which can lead to an early failure. A light trickle-charge should not hurt it, but it's not really necessary once the battery is topped off.

Would you share more details of the setup? What size battery are we talking about - 12 volts, amp-hour capacity? What about the charger - how many amps is it rated for, does it have different charging rates? Some chargers know to back off once the battery is charged up and therefore avoid any problems with overcharging.

Best wishes.

EDIT: BTW most SLA batteries will last between three and five years. Anything beyond five is a gift.