I think that the one month rule of thumb is for wet cell batteries and my understanding is that AGM batteries can last longer in storage (i.e. AGM batteries have slower discharge).
Correct. When we store our camper for the winter in indoor heated storage (October till May), I completely disconnect our fully-charged AGMs........and don't do a thing till May. Just leave in the camper. Our AGMs hardly drops in charge at all over this long storage period. If you have the old lead/acid batteries, all bets are off...they'll be pretty well discharged after this length of storage without careful maintenance.
I've been doing this since 2007 (when I got rid of those pesky lead/acids), and replaced with AGM-- at 10 years old, they are still nearly as good as new.
I once had to do all that battery maintenance during storage with lead/acids, but none of that for me any more.
This is my experience; your mileage may vary with different AGM manufacturer/model.
