it all depends on where you are...I already have a solid core from years of martial arts training, so I get by with 3-4 sets of crunch movements (either on a swiss ball or a delcine bench). I also think the heavy compound movements I do (squats and deads) tend to activate the hell out of my entire core as well. other than that its all BF % like Role said.
I see guys doing waaaay more ab work than they really need to all the time...I just never understood working your core out with set after set from all kinds of different angles when just doing a few basic compound movements will give you a huge amount of muscle activation in your entire core, and keeping your BF in check will keep it all visible. Hell, I've got a visible 6 pack at 11-12% and I put MAYBE 15 min per week in ab isolation movements.