Okay here is illustrated a technical fact where, I believe, cruisers err.
If you have say 6 golf cart batteries (6v) with 100 amp hour ratings each and have them paired to produce 12v, at 12v usage the total amp hour capacity is only 300 amps. If you have a 6 volt system yes you would have 300 amp hours. Amps time volts equal wattage. Am I not correct?
If you have six 6VDC golf cart batteries with a capacity of 100 amp-hours each, and pair them to get the equivalent of three 12 VDC batteries, with a total bank capacity of 300 Amp-hours... however, if your system is 6VDC, you would get 600 Amp-hours out of the same batteries... If you had both 12 VDC and 24 VDC systems aboard, you could get one 12 VDC bank (two batteries) with a capacity of 100 Amp-hours, and one 24VDC bank (the remaining four batteries in series) with a capacity of 100 Amp-hours as well.
Don't confuse amp-hours with wattage.
Also. AGMs can support 30 to 40 percent charge rate, so if you want to minimize charging time you will go with the biggest charger you can with AGMs.
Yes, AGMs can support more than 30-40 percent charge rates, but getting too large a charger is also not very cost-effective, unless you're battery management skills are pretty weak. All batteries, whether they're AGM or wet-cell, will accept a fair amount of current up until they reach about the 80% charge state, and then the charge acceptance level will drop like a rock...and you'll be float charging them until they are topped off. The current acceptance rate during the float charge portion of the charging cycle is relatively low.
Let's go with the original poster's 600 Amp-hour bank. Say you get a 90 Amp charger for it... You can use about 300 Amp-hours before needing to recharge it. That means you will need to replace 180 amp hours to get to the 80% charge state. This will probably take about 2–3 hours with this size charger—a larger charger might reduce this slightly. However, getting from 80-100% will take much longer—regardless of what size charger you have.