I leave a battery in the charger until the light goes green. Then remove. It will stay charged for months just sitting.
One thing to be aware of is that storing a lithium ion battery in a fully charged state will degrade the battery. Anytime you store a lithium ion battery, you should do so at 50% capacity (approx 3.6V). It's much better for the battery to give it a charge before use instead of leaving it in a charged state. Obviously that's inconvenient in mowing season, but it's not too much trouble to run it down to 50% before putting in storage in the off-season.
The hardest thing on a Li-ion cell is the upper and lower ends of the charge / discharge cycle. They degrade quickly if left at either end. When I say quickly, you'll probably get a year or two if you leave them in a fully charged state all the time. You'll typically get several years out of them if you can avoid storing them fully charged.