I was on the cleanse for 10 days (+ 3 days afterwards on just fresh juices). I lost 12 pounds and have since regained 6 pounds, but i'm stablizing now and haven't gained anymore weight. i'm sure most of it was water weight and food matter in my stomach and intestines. i could have kept more of the weight off for longer had I not gone to eating chocolate, peanut butter, breads, etc.
I do eat healthily in general (mostly fresh salads and fruits) but i do allow myself to indulge on chips and chocolate every now and then. If you do the Master-Cleanse and eat healthily (avoiding junk food completely), you will be able to keep all the weight off!