As others have written, you could probably lose 15 lbs in a 20- 30 day MC, but you won't keep it off unless you change your habits (and you'll probably gain back 3-5 lbs or so anyway because of the food going through your digestive system at any given time).
I use the Master-Cleanse for weight loss among other things, it is the only thing that has worked for me. But I know that any pounds I lose during the Master-Cleanse won't stay off if I don't do something different once I go back to eating.