Based on my knowledge on batteries (AGM), the initial battery charging (for ex. a 12V, 200AH battery and a 24V, 20 amp charger ), takes not more than 2-3 hours for the first 70% charge and then it takes like 4-5 hours to charge to 100%. Why is that the initial charging happens so fast ?
A 12 volt "Smart" charger will deliver its full rated current until the battery's voltage reaches about 14.4 volts, then will reduce the charging current to maintain 14.4 volts. After some time, the charger will reduce the charging current to allow the voltage to drop to a float charge level.
If you try charging a 12 volt battery with a 24 volt charger, the charger will deliver its full rated current for ever, as the battery voltage will never get up to the 28.8 volts the charger is trying for. The battery will eventually boil dry and be destroyed.