How long does it take for people to start looking anorexic if they only eat 300 calories a day?

It may take too long. The body will go into 'starvation mode' where it will hold onto all of the body's fat that it can and the person may die before they see 'anorexic looking' weight loss. The stomach then gets bloated and the person will look very pregnant. The the body will start digesting the muscles, then the intestines, which will kill you if your heart doesn't fail you first. It's not worth it. Lose the weight the way you gained it, gradually.