There is a difference of opinion in the fitness industry on this topic. Many believe, like myself, that the abdominals should be treated like every other muscle group in the body. In doing so, you would provide it with at least a 24 hour rest period in-between workouts. Like your chest or legs, it is not advised to work these large muscles groups to fatigue and then work them again the following day.
A varying answer to this question lies in how you define ‘do abs.’ There is a difference between inserting a set of ab exercises in-between every other set of strength training you perform (which could compromise of 9-15 sets total). Compared to doing a couple sets of crunches in a warm up or cool down after cardio, in which case your abs can be worked again the following day.
If you perform many sets of ab exercises and you achieve a state of fatigue then you will need to provide this muscle group with some rest. If you exercise on a regular basis your abs are technically working during every exercise you perform to stabilize your body. Therefore, they are not likely to reach an over-trained state with a set of 50 basic crunches in a day or a handful of planks.
A more important consideration is that along with ab exercises should come lower back and pelvic stabilization. In order to maintain balance between the front and back of your core, you should be inserting pelvic bridges and supermans into your routine every time you do a set of abs.
Here are some ab exercises that use an xering which is an affordable tool for additional resistance and forces more of the core to be involved.
The abdominals are comprised of several muscles:
the Obliques (external and internal) which are located on your side body,
the rectus abdominus, which is the center section of abs that most try to convert into a six-pack and,
the transverse abdominus, which is a deeper layer of muscle fiber that assists with complete function.