Battery questions are hard to answer.  Mine are on their last legs and I can play around for most of the day.  We have a large hill separating our property and this is where the battery gives up the ghost.  The "Curtis" fuel gauge suddenly stopped functioning and I must address that.
How long batteries last depends on usage and maintenance.  I bought mine at an auction cheap, but bought new plastic and OEM seats.
The downside is that JD thinks they have a license to STEAL on all things TURF!  I just paid $30.00 for a $3.00 Motor Bearing, not knowing it wasn't anything special.
An electric Motor is a few grand, as is the tranny.  Same for the controller.
Yesterday, I think I broke an axle shaft.  Electric vehicles have tremendous torque and at 4000 something hours, I guess the shaft just had enough.  Fortunately, I had bought an entire used drivetrain and controller for $500.00 some time ago.  I will just switch out the whole motor/axle assembly.  I really, really miss that machine when it's not in service.