Less than an hour first thing in the morning, less than an hour around mid-afternoon/evening. My father built the house I share with my sister, and it's ridiculously well-insulated. I can't have the heating on for more than an hour without getting overheated and needing to open a window - as it is, I always have the little top window open and the main window on the summer lock in my bedroom, right through the coldest parts of winter.
Edit: If it makes any difference, I've always preferred cold to heat. I've got a core temp slightly higher than average, and I get very warm very easily. Good for the girls, not for me.