I had one girl fast for 9.5 months and only lost 50g. It can be worrisome and obnoxious at times, but most of my snakes have been fine about eating year round, or fasting only for short, or commonly expected periods (i.e. females before laying eggs).

If it gets too cold, they can fast, if they get stressed, they can fast.. if they're breeding they may fast. Sometimes they just decide to fast once they get to a certain size to be annoying. lol.