§91.207 Emergency locator transmitters.
(c) Batteries used in the emergency locator transmitters required by
paragraphs (a) and (b) of this section must be replaced (or recharged,
if the batteries are rechargeable)—
(1) When the transmitter has been in use for more than 1 cumulative
(2) When 50 percent of their useful life (or, for rechargeable
batteries, 50 percent of their useful life of charge) has expired, as
established by the transmitter manufacturer under its approval.
The battery replacement is preventive maintenance per Part 43 Appendix A(c) and may be owner-performed (and must be logged). The annual functional check per 91.207 requires an A&P and is normally performed at the same time as the annual inspection and normally documented in the annual inspection logbook entry, although it’s not actually part of an annual inspection per Part 43 Appendix D.
Getting information on how long they must transmit is surprisingly hard. Many of the technical documents cost money to access, e.g. DO-204A, Minimum Operational Performance Standards (MOPS) for 406 MHz Emergency Locator Transmitters (ELTs) so I can’t read them.
I found one reputable source for the lifetime once activated.
“Depends on a lot of factors. The design criteria is that they transmit for at least 48 hours at 0-degrees, some will transmit much longer if the weather is warm, the battery fresh, etc. Or they will transmit for less if it’s colder, if the battery hasn’t been replaced when it was supposed to, etc.” Richard A. De Castro -N6RCX NREMT SAR Tech