The answer? Once a week. Now before you get all worked up and think that it is impossible think about it. Your abdominals are muscles just like any other muscle group. Now let me ask you a question. How often do you train your legs directly? My guess would be once a week (well I hope so otherwise you are
overtraining or not training them at all). Do you believe that training your legs every day would make them bigger, or for that matter any other body part? No, of course not! By now I hope you have learnt that muscle growth occurs outside the gym while resting - the gym only provides the stimulus for muscle growth.
What happens if you train your legs or chest everyday then? Well they never get a chance to recover and grow and you end up without the desired results. So what do you think will happen to your abs if you train them every day? Yep you guessed it - they never get a chance to recover and grow.
I literally train my abs once a week for around 15 minutes. That is all your abdominal muscles need. Making them pop out and become a lot more visible you need to drop your body fat levels down fairly low. That is when your cardio and diet come into play.
A shredded set of six-pack abs is the result of well-developed abdominal muscles and lowered body fat levels. No ifs or buts about it. Without a low body fat percentage your abdominals would not be visible: they will be covered with a layer of fat. And without developed abdominal muscles you won’t have a visible set of six-pack abs even if your body fat is low.
To reduce your body fat you must create a calorie (energy) deficit. What that means is the energy (calories) you take in must be less than the energy (calories) your body uses each day.
For you to create this calorie deficit you must do one of two things or both. You must either expend or burn more energy than you consume or you must consume less energy than you expend. This is achieved by exercising or eating less. So training your abdominals every day will not in any way cause you to burn more fat in your mid-section faster.
It sounds too good to be true, but the concept of building muscle does not change from one muscle to the other. No need to kill yourself doing crunches everyday to get those washboard ripped abs. Save yourself a lot of heartache and time by keeping your diet and cardio in check and attain those abs faster.