Here is a more technical reply.
Your battery charging speed depends on how far your battery is discharged and how much current your converter is pushing thru your battery.
Obviously, if your battery is not very discharged, it will charge faster than one which is fully discharged.
The other factor is determined by your converter. Many manufacturers install an inexpensive single stage converter that charges at 13.6v or so and that limits the charging current to about 5 Amps. It also can cause the water to evaporate when the battery is fully charged.
Much better is a "smart" charger. (not a brand name). A smart charger senses the battery's state of charge and sets the charging voltage accordingly. a deeply discharged battery will be charged at about 14.2v, depending on the generator brand. The charging current can be in the 30-60 Amp range, depending on the battery and converter. Once the battery becomes somewhat charged the charging voltage drops to about 13.8v, as I recall. Once it is fully charged the voltage drops to 13.2v, which is much easier on the battery than the 13.6v mentioned above.
I think most of the replies above are for a smart charger.
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