Normally they take around 3½ to 4 hours to recharge, but sometimes the charger fails to cut out when they're done, so when I think they've had long enough I reset the charger and it reports that the Batteries are fully charged.
Last night I put them on charge and fell asleep so they were charging for about 9 hours and quite warm to the touch when I switched it off this morning.
They've gone back in the scooter and everything seems fine, but have I done any damage?
If everything is working OK then you have probably not done them any harm, if they are not sealed unit type batteries it may be worth while checking the electrolyte level in the battery cells as some may have been lost due to evaporation when the batteries got a bit hot.