Source: Battery University.com is a great source of battery info.
https://batteryuniversity.com/index....d_acid_battery
Fully charged batteries are good for up to 6 months.
A quote from their site:
"Lead acid batteries must always be stored in a charged state. A topping charge should be applied every 6 months to prevent the voltage from dropping below 2.05V/cell and causing the battery to sulfate. With AGM, these requirements can be relaxed."