I would say a healthy goal is to lose 1.5-2lbs. Anymore then that and you will be losing muscle. Keep in mind if you are very over weight then those numbers can change. I have seen really fat people lose 5lbs/week but they are not worrying about sparing muscle loss. If you are already slim, around 15% BF and want to cut down to 9% or so then 5lbs a week = muscle loss guaranteed. Remember 1lb = 3500 calories so if within every day you are at a 800 calorie deficit then that adds up to about 1.5lbs lost per week which is very attainable.