Batteries used in the ELTs must be replaced (or recharged, if the batteries are rechargeable):
When the transmitter has been in use for more than 1 cumulative hour
When 50 percent of the battery useful life or, for rechargeable batteries, 50 percent of useful life of the charge has expired
An expiration date for replacing (or recharging) the battery must be legibly marked on the outside of the transmitter and entered in the aircraft maintenance record
This does not apply to batteries that are essentially unaffected during storage intervals, such as water-activated batteries
https://www.cfinotebook.net/notebook/avionics-and-instruments/emergency-locator-transmitter
everything you need to know about emergency locator transmitters/