Lead acid batteries start to sulphate as soon as they come off the line
But seriously there used to be a chart for floodies that showed the rate of sulphation at a given state of charge, looks a lot like a bell curve once you get past 50% DOD.
This bell curve can be fixed with a C200 or better charge. So a little tiny solar charger would fix your sulphation issue almost but not quite completely.
AGMs are much less prone to low DOD sulphation as are gel, so I wouldn't worry about it too much, my car afterall sat at 75% SOC everyday for 8-10hrs without issue with many excessively deep discharges.
Good Luck
Ryan