It's best to train abs with short high rep workouts every other day or 3 days per week.
The goal when training the abdominals is completely different then when training other body parts. When training other body parts like chest, back, legs etc. the goal is to make those muscles bigger and larger.
That obviously isn't the goal with abdominals. With abs, the goal is to define them and keep the midsection tight.
You want to train abs with high reps and to train them frequently to tone the midsection and keep the midsection tight.
This is the template I recommend for ab training:
super set 1 : 3-4 sets of lower ab exercise x 20 reps super set with upper ab exercise x 20 reps
super set 2: 3-4 sets of lower ab exercise x 20 reps super set with upper ab exercise x 20 reps
3-4 sets of an oblique exercise like decline bench romanian twists - 20 reps per side
1 set to failure of a functional ab exercise like plank or ab roller