First let me start off by saying no amount of ab definition will show unless you lower your body fat enough to show them! But…good news is the more ab muscle you have the more they can show at a higher body fat percentage, score!
Abs are just like any other muscle they need resistance in order for them to grow. Instead of only doing bodyweight exercises; try adding weights to your ab routine, this will help create more stimulus and also muscle growth.
For example when in plank position add a weight to your back and try to hold it for as long as you can. When doing crunches hold a weight in your hands and crunch up with it. Adding weights to your ab routine will significantly make a difference. ADD HEAVY WEIGHT! Train your abs the way you would train your legs with intention for them to grow!
I wanted to add that not every body wants the chiseled ab look, meaning, they prefer a flatter stomach that looks naturally lean. For this person I would recommend doing body weight ab training just to naturally stimulate the area without trying to add growth stimulus. However, the downfall when you don’t have rock hard abs is when you gain body fat your abs go away very quickly.
Take me for example I purposely add weights to all my ab training because for one I love the way abs look with they pop and second if I ever gain some body fat I will still look rather lean. This is because I have created larger muscles in my abdominal area that it will take more body fat to cover my abs. Score!
How many times a week to train abs?: For health purposes you only need to train then 1-2x/week this keeps your core and back strong. However if you train them 3-5x/week you’ll notice a much more defined core given that you are eating properly to let those abs pop.