My XR charges within 4 hours (when it switches to green) and that is the point where i want to disconnect it.
Charging speed depends on current and when the charger slows it to trickle to preserve battery life (my scooter does that by slowing down around 80-90% when it really takes long to get to the last percentage points).
If you get the full range within 4 hours, i would not let it supply voltage for another 3 hrs.