At most, not least brain fart. 3% is the bare minimum, you body won't allow anymore loss after that. Unless you have an insane amount of muscle mass; which is plausible but not very common.
Those are the numbers I hear from coaches, nutritionists and trainers. You can go around and ask for it. You can find some material on this at wikipedia
http://en.wikipedia.org/wiki/Body_fat_percentage and scroll to recommendations.
But like I said, that's the standard I go by - my safe zone. I know if I ever went up to those ranges (like 13) I would start doing something about it. At 10% and under, you are in the 6 pack range. At least to put it in some physical perspective for some here.