Men: To make your entire six pack of abdominal muscles visible, you'll need to get your body fat down to somewhere below 11%. If you get it down below 8%, you'll almost certainly have a clearly visible six pack.
Women: You need to get your body fat down to somewhere under 17%. 14% would be even better. These are higher numbers than the guys have to hit, but remember that you naturally have a higher fat percentage, so your job is just as tough as for the guys.
Those numbers are difficult to reach, but not unattainable. Depending on whose results you use, the average American male has a body fat of between 17 and 26%.
The average American female has a body fat of between 22 and 36%. You have your work cut out for you, so make sure you pick a solid program and stick with it.