By having at least a days rest in between direct ab workouts, you are allowing the midsection to recover and grow just like your other muscles. It is best to train your abs after your regular workout, and stick to the time frame of 10-15 minutes at most. You do not want to spend 30 minutes doing straight abs exercises. Simply choose 2-3 exercises and cycle through them 2-3 times each. This will be far enough to get you a great workout.