You are using the tender, GOOD DEAL !
My stock battery '97 lasted till ealier this yr, that is over 12 yrs.
Using the battery tender is what makes the big difference. It reduces the build up of sulphate and even to some extent will break down light sulphating and extend the battery's life accordingly.
Keep using the tender and don't worry about it till it is showing signs of capacity loss, and, the only real way to determine the capacity of a battery is with a load test meter.
The biggest killers of a battery are:
cooking or leaving a constant charger on too long.
Drawing down too far below surface charge & leaving it.
Freezing, a low battery will freeze sooner than a full charged one.
These things will cause a deep sulphate that the tender can't deal with.
Also note that many of our MC batteries have a soft lead terminal (rather than harder alloy) which can loosen with vibration of MC and need perioding tightening (causion with tightening) and over time the terminal can get crushed beyond useability, Mine was close to this state when it lost its capacity as tested with a load meter !
'97 VN1500 91 Cubes Of Custom CLASSIC
Vance & Hine 2to1
engine, forks, ignition & carb mod's
WebCam cam & rockers hard weld & grind
NEW TOP END, NO LOOKS ALL GO GO
Maxx intake manifold system & super G carb 44mm