Check out this great video from Sean Nalewanyj discussing how often should you train abs.
How many ab workouts should you perform per week for the best results?
First off, if you really want to maximize the visible definition of your abs, then yes, direct ab training will be necessary.
Big compound lifts like squats and deadlifts will not be enough to develop the rectus abdominus (the “six pack muscles”) to their full potential as some people claim, and isolated exercises like weighted crunches, leg raises and planking variations will be necessary for that purpose.
So how often should you train abs?
The simple answer is to train them at the same frequency as all of your other muscle groups.
The abs are a muscle just like any other and they respond to training in the same basic way. For that reason, there’s no need to train abs every day or 5 days a week like so many people do.
For a typical natural trainee, this will generally mean an ab training frequency of 1-3 times per week, with twice per week being the general sweet spot I recommend to most people.
The only situation where a higher ab training frequency would be necessary is if your abs are a specific weak point that you need to bring up.
In that case, your ab workout frequency could be raised to 3, 4 or even more times per week to see if they respond better that way.
However I’d only recommend this if you’re well past the novice stage of lifting, you know your body well, and you know that your abs are legitimately a lagging muscle group. (And the only way for you to know that is if you’ve dropped your body fat percentage low enough for you to visibly see that your abs need more work)
In all other cases, the question of how often to train abs is very simple, as there’s no need to train them any differently than your other muscle groups.
via Sean Nalewanyj