How long can a battery possibly last

If it isn’t a maintenance free battery and you can open the top, a battery will last as long as you can refresh the acid and clean the plates, Until the lead plates are gone and as long as the acid is refreshed, it will continue to operate.

Maintenance free batteries don’t allow that to happen, so they only last until the lead chloride covers the plates and the acid is depleted shutting down the electrolytic reaction that creates electrical power.

Keeping a battery well charged prevents the depletion of the acid and deposition of lead chloride on the plates.

The best way to keep a lead-acid battery alive it to keep it well charged.