depends what your looking for
for me i want more progress then just other compound lifts, so i do a separate workout. I usually do around 5-6 total sets of weighted crunches, weighted sit ups, leg raises, or cable crunches. Me personally, i do these sets differently from other muscles. Instead of doing reps from 6-8(strength building for me) i pick a weight where i can hit about 15 reps then failure. I dont know for sure, but from what i understand about muscles, doing the higher reps with an easier weight will produce tone muscle, which is what im looking for, rather then strong muscle.
Heres an example of my ab workout:
Leg Raises - 2xFailure
Weighted sit ups - 2x12-15 35lbs
Weighted crunch - 2x15-20 20lbs
if you do this right, at least for me, your abs will be screaming and most likely very very soar, but hey that means its working. and thats IMHO on abs.