I'm afraid this is a 'how long is a piece of string' question. I have tried 5:2 which is in effect a 12hr fast combined with VLCD. That worked as far as weight goes. I now only have 2 meals a day so in effect and IF regime of 16:8 hrs or more nearly 17:7 for me, and I have done a day of 24hrs between meals, and I might repeat that soon