That is totally up to you of course, but you can do that as often or as infrequently as you like. I, for example, do either a 20 to 24 hour fast more or less pretty much daily. For me, that suits me and works well into my schedule. Some people would much rather do it once or twice a week and that works great for them. When I first started with IFing, my plan was to do it once or twice a week myself, but then it quickly went to three times and then every day.