I've only ever done intermittent fasting and tbf most of the fasting time was while I sleptIt depends what your fasting for some people do it for weight loss, I do it for hormone levels with PCOS. A fast won't 'cleanse' your body that's what your liver and kidney do. Yes you should ideally work up to something like a 3 day long fast there are loads of people who do 16:8 or even 20:40, I've never done a multiple day fast myself because to me it's madness