I think it's pretty strict, at least when there are still quite a lot of boats around. Not sure if that's the case in the winter on all the moorings but, even in winter, I've noticed that forum members take a dim view of boats that hog the 24hr moorings that have electric. However, a few weeks back, when we had those awful strong winds, I posted about being worried about moving the boat during the storm. The helpful BA spokesperson who sometimes posts here said that the BA would allow boats to overstay the 24hrs in those circumstances, as they wouldn't want people moving their boats if they thought it unsafe to do so.