We have basic flooded golf cart batteries and don't plug the boat in during the summer. Typically, we run the boat enough to keep the batteries charged up but didn't this year because of family stuff.
I don't recall how long the longest interval was between going out on the boat but six weeks sounds about right and the batteries were fine. Never got below 80% I think, and we have a little computer type fan running all the time through the desiccating head.
"The most interesting path between two points is not a straight line" Murray Minchin