If you only take in 800 or less calories a day how fast will you lose weight being 5'5 And 128 pounds with no exercise?

No! Everyone on a diet reach their own plateau. It will kick in again. Women require 1200 calories a day to be healthy and exercise is a must! If you don't use it you lose it and that includes the heart. If you exercise then you know muscle weighs more than fat, but what many people don't realize is when they pack on the required amount of muscle for their body size they look great and even though they hoped to be approx. 120 lbs. - 150lbs. for a woman, and may be 135 lbs. or 180 lbs. take a look at the muscle. Muscle need not bulge! It makes the person look lean and mean. LOL