I don't know how long it took because it was unattended. When I came back it was at 13.6v with the charger on. I don't notice a change in the water level. I removed the charger and will let the batteries settle down 24 hours then measure resting voltage. Meanwhile I hooked it up to my son's '99 Miata (which has taken over my garage). (Nice when your kid offers to clean your garage, eh?) (grin)