Well, I only know it from a man's standpoint - males usually need to hit single digit bodyfat percentage to see defined ab muscles. Depending on your genetics, they might show up at 12%...but for others it might not be until 8%.
for women it is higher, as women naturally carry more body fat. Off the top of my head, I think its like 14% or so when they should see abs.
Also bear in mind that some poeple just do not have genetically large ab muscles, and won't ever have a wicked six-pack.