Well, you can tell about how much you will lose.
Go to calorie calculator and find out how many calories you burn daily.
Multiple this number by the amount of days you will fast. (Ex: 2,000 calories burned daily x 14 day fast = 28,000 calorie deficit)
Divide this number by 3,500 (One pound of fat is 3,500 calories. Ex: 28,000 / 3500 = 8lbs lost in 14 days of fasting)
This will tell you roughly about how much weight you will lose. (Assuming all weight came from fat and not muscle)