Theoretically, you could lose as much as 20 pounds a week. However, much of that weight could be water weight. That means that, once you go off your diet, you are likely to gain much of that weight back. Also, unless you engage in strength training, you will be losing muscle as well as fat, since about ¼ of the body's weight consists of muscle. It is interesting to note that, at most, you can probably lose four pounds of fat in a given week.