I would say ab muscles are no different than any other muscle....if you work them hard, then you need to give them sufficient recovery time. I know, when I have worked mine in the past, they can get sore and i wouldn't dream of working them the next day or any other day until they are
fully recovered...just like any other muscle they can become over-trained. I would say that anyone
directly working their abs everyday is not working them the way they should or could be (as in not hard enough!).
Work them hard with 100% intensity and then let them rest....keep a log on how long it takes for yours to recover and then work them again.....after a few weeks you'll be able to look at your log and see exactly how long it has taken your abs to fully recover between workouts. Now, everyone may be different on recovery time, as it all depends on genetics, how good your
nutritional intake is and how much sleep/rest you give yourself...all this can hinder or speed up recovery time with all muscle groups.
I hope this helps some!