Abs, unless overtrained, can typically be done every other day. The question is WHY?
Personally, I never train abs, unless I am training for a show. When I get my BF low, I can always see my abs, they are constantly being trained with the heavy weights that I use.
Unless you are very low BF, I see little reason to train abs. If you are in need of functional ab strength, then training them heavy once per week is plenty.