I have to dissagree with you Ken, -"IF"-your wallwart charger is charging at 100ma as stated and assuming you never run a battery dead an overnight charge (12 hours) is plenty. That said, be carefull about stated charge rates.
Some chargers, like that wallwart, state a 100ma charge rate but may split that when charging 2 batteries at the same time, so in effect your really only getting a 50ma rate, in which case Ken is correct. You need a better charger if you want overnight charging.