My understanding is that to start getting the metabolic benefits of intermittent fasting, it is best to fast for at least 13 hours which is usually achievable by extending the overnight fast. I've also read that stopping eating prior to 7pm is beneficial. Other than that, it's best to play around with different regimes to find the one that works best for you. If you have a lot of weight to lose, fasting every other day is one method. There are all sorts of ways of doing it - 24 hr, 36 hr, 48hr, etc. There is also compressed window of eating eg. 8 hrs eating/16 hours fasting.