All the time with solar charging, which is temperature compensated (required charge voltages go quite high in cold weather). Not claiming expertise but things I've read don't advocate to let them sit for more than two or three months without topping them off. I think the only reasons to take them off the boat is if your charger isn't temp-compensated, or as a safety precaution -- if they're allowed to run down too low the electrolyte can freeze and crack the casings. If I didn't have the solar panels I'd still pick a warmer day every month or two to throw a charger on for perhaps 6-12 hours to top off.