Charging your Waterpik up after the battery has gone low takes around 4 hours to give the Waterpik battery a full charge.
Your Waterpik has a blinking light when it's charging and when that light which is a charging indicator light on the Waterpik stops blinking then the battery for the Waterpik is fully charged and you can unplug it.
I always just plug in my Waterpik after use to keep it charged and have it ready to go and then usually after I go back into the bathroom later it's charged and then I unplug it.
So just plug the Waterpik in and let it charge and check on it a few hours or so later and once the batter charging light has stopped blinking the Waterpik is charged and you can then unplug it.