First of all, it's crucial to know that no matter how hard and how long you workout your abs, they will not show, unless you've gotten rid of the excess belly fat, exercises strengthen your abdominalsand give them a more well defined shape, while cardiovascular activities (walking, running, intense weight lifting) burn body fat (everywhere in the body, you cannot target a specific area) and allow those muscles to show.
It really depends on how hard your train your abs, light workouts justify doing it daily, since little strain is put on the muscles and little time is required for recovery, but this regimen will bring you only this far, like in bodybuilding, you won't go far by lifting the same weights all the time, you need to attack your muscles with growing force for them to grow! On the other hand, there's working out your abs vigorously, until you can no more, this is the way to go to strengthen your abdominals, in this case, a full rest day between workouts is a must, since during rest is when muscles repair and grow.
Take into account, that by working the same muscle group repetitively day after day, you risk injury, to avoid this, perform different exercises or as was suggested above, instead have vigorous exercises 3-4 times a week with a full day rest after each workout day.
Doing different exercises not only helps prevent injuries, but is also the way to go, since you need to train both the abdominals and the lesser known obliques (the muscles to the sides). Exercises for abs would be: crunches & leg lifts (lie with your hand behind neck, lift legs to form 90 degrees angle and lower, all in a slow manner); exercises for obliques would be: Russian twists (sit with your legs forming an arc above the surface, torso at 45 degrees angle, hands to your chest, or stretched forwards if you hold a weight, move from side to side) and bicycle.
Working your abs hard doesn't mean doing hundreds of repetitions, but doing more intensive exercises, those that are hard to perform and won't allow you to do as much repetitions, basically anything other than the first few exercises that come to your mind, such as crunches and sit ups. Intensity is key, and don't forget good form.
Conclusion
The bottom line is, if you are in serious need for a six pack, you need to workout hard with a full day's rest afterward, doing different intensive exercises and doing them slowly, until you hit fatigue. Remember, if you don't feel that burning sensation in your muscles, you aren't training them hard enough, no muscle tearing is occurred and in turn no growing/strengthening process. One more thing to remember, is to shed off that extra fat around your stomach, or else no matter how defined your abs are, they will be hidden behind the curtain that is FAT!