When the battery is fully depleted, it takes around 12 hours to charge to full capacity.  The scale comes with overcharge protection and will stop charging automatically when full. However, it is not recommended to leave your scale plugged into the charger when fully charged for excessive periods of time (i.e. more than a day or two).
Note: It is best to start charging when there is only one bar left on the battery indicator, instead of waiting for zero bars.