I've done several fruit juice fasts for over 10 days. The key is to keep the bowels moving and make sure that you stop the fast or slow it down, if you start to feel too sick. 2-3 days is better than nothing, but I believe that the body only really starts to cleanse after about 10 days.
Even doing a daily fast from suppertime until at least noon the next day can make a bit difference IMO.
I haven't done a water fast, as I believe it would cause a person to detox too fast and could be dangerous.
It's a good idea to read up on it and one of the better authors I've found is Arnold Ehret. I also like Stanley Burroughs "Master Cleanser" and MH's Book 1.
There really aren't any hard and fast answers. Care must be taken to listen to what your body is telling you.