They (Extra) recommended changing the battery every two years since they
seem to know the history of the typical battery life (cranking the IO-540),
and the fact that it is much easier to change the battery (an RG35XC) during
an annual rather than waiting for it to die. (It involves taking all of the
panels off of the plane as well as the front seat, so is much easier to do
during an annual).

In my much smaller Decathlon the battery seemed to last around three years
before needing a change, so this sounds about right to me.