"It's very individual" - Eric said it right.
For males, anything in single digits would work, generally ~8% bodyfat (depending on one's height). For females, this varies so much - I personally can't see my 6-pack until I'm ~8% bodyfat, whereas other women can see it at 12-15% bodyfat.
Other factors come into play as well, such as height (taller people are going to look leaner than a shorter individual at the same body composition), genetics (some people simply cannot get a 6-pack), how much muscle mass you have in your midsection (the more muscle you have there, the more likely you will see it at lower body compositions).
If you want your abs to "stick out", you need to make sure you build up the deep grooves, and this happens from doing weighted exercises for them. In saying that, not everyone who does that is going to have them either, since, as already stated, genetics plays a large factor in what your abs actually look like once you are lean and have built the muscle mass up there.
~Rosie~