About a week. Unless you drink lots of water, it will be mostly water weight loss the first week. After that, it will speed up, then after a longer while, it will slow down because you have ruined your metabolism. If you starve yourself for just a few weeks, you're not anorexic yet, you are just dumb. If you continue on this path, you will eventually classify as anorexic, but it takes a significantly underweight person to be an anorexic.