You will loose weight on the MC. How much depends on many factors. When you fast it gives your digestive system a break and the body will try to repair itself and use the extra energy to do what it needs to do the most >>first<<. Don't count on loosing a specific amount of weight cause you will be disappointed if you don't.
Fasting is not the best way to lose weight over a long period of time. However, fasting does cleanse the system allowing it to function better and assimilate more nutrients from less food. It just isn't a diet, its a tool to make a correct diet more efficient. Try to look at this as a cleanse, not a weight loss plan. When it is over you can enjoy the benefits of a healthier body while you lose weight safely and effectively.
The Master-Cleanse is NOT a weight loss diet primarily. It is a as the name implies a "Cleanse" and it's primary function is to cleanse and heal. The Master-Cleanse or any other "fasts" are not a good way to loose weight >>>>UNLESS<<<< you are committed to long term maintenance afterwards because when you fast your metabolism slows down considerably and if you don't follow up with a good solid healthy Diet and REGULAR exercise afterwards you'll very likely gain most if not all and possibly more than you lost on the fast.