It depends on what you mean by most effective. Most effective for what? For weight loss? You want a pretty lengthy fast to get into those deeper levels of ketosis. If you just want to lose some eight and get the benefits of increased autophagy, then 3-7 days should suffice. Even a 48 hour fast can give you good benefits. If you don’t go past 7 days you should be ok as far as refeeding goes. You definitely want to get your electrolytes, and if you’re concerned, get some thiamine and phosphorus to take shortly before you break your fast. I’ve never had a problem though coming off a 7 day fast except one time when I broke it with a really fat heavy meal. I was in misery for hours. Take your electrolytes and you should be fine. Keep up the good work. You’re doing great.