It is all up to you, your body, and your diet history.
There are some people who 'attempt' the Master-Cleanse (don't really follow the recipe and the DOs and DON'Ts) and then complain because they didn't lose much weight or couldn't resist temptations (and eat) or had a headache.
Some new MCers are in a constant state of fear before and in the beginning. That fear weakens the immune system and might make your body hoard fat...So relax. You've skipped meals before. You will be amazed that the lemonades will satisfy your hunger. The tricky part is breaking the emotional and social habits of eating. The body sometimes adjusts quicker than the mind. (Another reason the fast is at least 10 days long, to break those cravings.)
Some people will have a rough first week because of addictions (coffee, smoking, sugar, white flour, etc.). In those cases, the body had been used to a certain diet and had built up a buffer, expecting the diet to continue. When those foods (toxins) stop coming, the body might rebel with headaches, runny nose, etc. As the toxins are flushed away with the lemonades, the body will start releasing the excess fat. (The body hoarded that fat to store those toxins so they wouldn't harm other parts of the body.)
Since the toxin amounts will get smaller and smaller, more and more fat will be released.
(Another thing to keep in mind--when you get off the Master-Cleanse successfully and cleared alot of junk out of your system, your body won't have that buffer (fat) in there. So if you go back to eating JUNK again, you'll probably have the headaches and stomachaches again and then your body will build up a new layer of fat to protect itself again. A vicious cycle.)
There are a very few people who are actually allergic to some ingredients of the Master-Cleanse (lemons, maple syrup, cayenne). So, at least a juice (veggie and/or fruit) fast would probably be better than nothing.
Read up on the B&Ps info in my recent posts. I think they work very well with a fast.