It depends on the current rating of the battery charger. Say you have a 100Ah battery, 50% discharged. A 2A battery charger will take (50Ah/2A =) 25 hours to fully charge the battery. A 10A charger will take (50Ah/10A =) 5 hours to fully charge the battery. That’s a rough indication. Other factors affect charge time:  Ambient temperature. A smart charger will decrease charging current as ambient temperature increases. This protects the battery from overheating.  Condition of the battery. A smart battery charger will make every attempt to optimise the battery’s charge level. So it might take longer to finish off the charge, as it attempts to restore the battery to its optimal state.