How often do you need to "Float" the batteries? Well, once they fully charge your converter should switch to float mode, So "Every time you charge them".
Now if you meant equalize... Only when they seem to need it, How to tell that is a bit complex. But for flooded wet cells, when the hydrometer shows discrepency cell to cell.
There is some discussion about equalization affecting sulfaction (Make sit less of a problem) so perhaps once a year for non AGM batteries.. AGM's go to the manufacturer's web page and check with them
One thing.. Do not worry about six or 12 volt differences.. Two six volt batteires in series (properly paired) are the same as a 12 volt battery, Treat them EXACTLY as you would a big 12 volt battery, think of them like a 12 volt battery, This answers nearly all six/12 volt quesitons.. You only have 12 volt batteries.. Some of them are 2-parters.
Home is where I park it!