ok this question is heard a lot.
I started working my abs 6 days a week, it was hard and I have done every workout 500 crunches and 500 reverse crunches.
it was madness and it was bad.
now I work my abs 3 days a week, like every other muscle in my body and I work a veraiety of exercises (11 exercises) and I do 2 sets of each exercise and 10-20 reps and I see a big improvement.
so in the end work your abs between 3-4 times a week not less, not more it can hurt you.
do only exercise you like so you will realy do the exercises and give your abs at least 1 day rest between every workout.