Well I think the general dogma preaches 0.5% bf / week MAX. So if you weigh 200 lbs. then you can lose 1 pound / week of pure body fat. Now.... remember that alot of the weight loss you see on the scale is water weight and a mass loss from depleted glycogen, a few pounds from an empty stomach that is otherwise full, etc. I mean, if you are 200 lbs. and 15% bf then you only have 30 lbs. body fat on your body. If you lose 20 lbs pure body fat you'll be at 5% which is pretty much getting down to your healthy minimum.