Its difficult for most people to lose over 2lbs of body fat per week unless you are significantly overweight. Usually when we drop a pound its a combination of a little bit of water and mostly fat and sometimes a bit of lean muscle mass which as much as we can try to prevent will most likely still happen to some degree even very slightly in most of us who have a good amount of weight to lose.

1.5 - 2lbs a week is a good pace to lose for most people who have under 30 or so lbs to lose not counting the first few weeks which is usually a lot of water weight.

Obviously there are no absolutes and some people lose more than this and people who have significantly more fat to lose will more faster but 1.5 to 2lbs of fat to lose per week is good loss and I suppose for someone who has little to lose it would be very difficult for the body to lose weight this fast without being in some extreme circumstances. It would be a lot of work for the body and may even shock it to some degree. Also for many weight loss faster than that would result in a lot of loose skin because the body isnt adjusting right.