I only work abs twice a week. I do weighted ab work at the gym (4 sets 15 cable crunches, 4 sets 15 regular crunches w/a plate on my head), and what I call a "core day" in which I'll do pilates or Core Max or something like that--I guess those workouts are 30-60 minutes, but work a variety of muscles as opposed to the traditional exercises above.
Overtraining a muscle will shrink it, & if you want the proverbial "six pack" you have to build the muscle, therefore you should work your abs the way you work any other muscle.