I've had my Kalkhoff Pro Connect for nearly 10 years: bought from 50-Cycles in Loughborough about August 2009. I also bought a spare battery at the same time.
The bike is a leisure rather than commuting bike, and I tend to use it only between April and October. I estimate that I use it about 60 days a year, doing at least 30 miles on each of those days. My "bread and butter" rides are in the Derbyshire Peak District; hills included, not just the flatter ex-railway trails.
I always aim to fully discharge the "working" battery by the end of the ride. (I nearly always carry the other battery as a just-in-case back-up.)
I fully recharge the discharged battery on my return home and also recharge the other if I've had to use it.
On the next ride I use the latter battery and repeat the process, thus cycling their use.
Generally, I try to use the lowest assist as much as possible, particularly in the first half of the ride. I use the two higher assist modes for steeper hills and when I'm kn******d.
To get to the point: the batteries still provide me with plenty of charge after 10 years. I (hopefully) attach a recent, slightly shorter than usual route that I did using a bit less than one full battery charge; deliberately running the battery down on higher assist on the return up the Manifold Trail.
p.s. The bike is showing its age now. I don't bother locking it up when I pop into a cafe, shop or pub; just make sure I have any valuables, including the car keys and battery key!