although diet is a must, as you have to get your bf down to be able to see your 6-pak, you still have to build your ab muscles just like any other muscle. i do abs every 3 days, but i do mostly weighted routines, so they stay sore for a couple of days. If your doing crunches i believe every two days would be ok, if its working for you. Abs are like your calves they require more frequency training as they get used on a day to day basis.