Only use the charger that has been supplied with your mobility scooter or powerchair. Using an incompatible battery charger is likely to be detrimental to battery life. If you lose your battery charger, be sure to replace it with one that's approved or supplied by the vehicle's manufacturer.
It will take around eight hours to charge your batteries to their full capacity, and while it's impossible to disconnect them from the charger the moment they're full, it's important to not leave them connected unnecessarily. As soon as you know your batteries are fully charged, disconnect them. When you remove your mobility scooter from charging, firstly turn off the mains and then disconnect the charger from the vehicle.