The longest I’ve detoxed and the longest you really should detox all in one stint is about 7 days.  Frankly, people have tried what they thought was negative calorie dieting which turned into detoxifying, because all they were eating at any time was fruits and vegetables, and this is really good for your body.  It really cleans it out and gives you that effect in general.  The trick is not to overdo it. 

By this I mean if you do it for too long, your body gets starved and it gets too used to it.  So you stop cleansing, your body starts going in reverse, and frankly it counteracts all the effects you are looking for.  So you can really only detox for a certain amount of time. 

I have seen some people who have gone for some 21 days, of course they haven’t been doing a stricter juice diet or anything like that, though I wouldn’t have put it past them to try at any given point in time.  But they were on diets that were only healthy for short period of time being used as a cleanse.  It’s one of those things that misunderstandings often abound.