How long should it take?

Hello! I am in university and have put on the "freshman 15" and I'd like to get rid of it before I go home and have to see everyone. I also have alot of stomach problems where I can not eat anything spicy, lactose, caffine, or greasy food without being in pain after (don't ask how i managed to gain weight) and i have read that fasting can help with that as well. I am 18, 5'4'' tall and 133 pounds how long do you think it will take me to lose around 13 pounds if I drink a small amount of juice, broth, and tea? (Probably no more than 400 calories a day) any information or advice would be greatly appreciated.