Whilst not strictly comparable the Li Ion cells in the Lange
Antares 20E were initially expected to last at least 10 years of
normal use. Currently there are plenty of Antares 20E's flying
around with their original batteries. I think the batteries are
now expected to last 20 years? (Maybe wrong about that). Of
the 72 cells in my Antares just 3 are now down to about 87%
of their original performance; the batteries are now 11 years
old.

For FES systems, where the batteries might not be used at all
on many flights, I'd expect a similar, or better, battery life
(assuming they are not abused of course; see earlier post).