Since I have been in this house, my swim season has run pretty consistently from mid-March through late October or very early November. On either edge of the season, a particularly strong cold front may keep me out for a few days but otherwise it is rather steady during that timeframe. I should qualify this by stating that swim season for me is when the water is 65°F or higher.
BTW, really don't use the heater that often for the pool. Do use it for the spa, though.
BTW, really don't use the heater that often for the pool. Do use it for the spa, though.