You CAN work your abs quite frequently, because, like the calves, they are muscles that have high endurance. However, how and how often you work them depends on your goals. Bodybuilders, who want very chisled, defined, almost protruding IMO abdominals work the less frequently and use resistance (weights or cables). Some "bikini models" work them daily with high reps.
One thing to think about: if your abs are your problem area, you can do as many crunches and ab workouts as you want but (despite what the infomercials for ab devices suggest), you won't see much of any definition without having a low enoough body fat level to strip away the layers on top. Many people have strong ab muscles, covered in a layer (or two or three!) of fat. Also, for women especially, the lower abs are often a prefered fat storage location for any excess fat.
I've always had somewhat of a lower belly pooch, but if I personally were to get a low enough body fat percentage to have a 6-pack, I think I'd be way too thin and bony on top!
In addition to abdominal workouts (or core workouts, that work the entire "corset" area, to tone the muscles and strengthen the core), cardio workouts (to burn caloris), and full-body weight workouts (to burn more calories: doing just ab work doesn't burn a lot!), you need to watch what you eat. There is a saying that "great abs are built in the kitchen," which stresses the importance of how you eat to how your abs look.
One good addition to any ab workout is pilates work, which works the transverse abdominals (that act like a corset holding in the lower abdominal area). The latest issue of Women's Health shows a move that works this muscle quite well : lying on your back, with legs off the floor, bent at a 90-degree angle with lower legs parallel to the floor and a yoga block (a pillow would work well also) held between the knees, do a reverse cruch, and each time you crunch up, exhale and do a kegel, which engages the transverse abdominals as well as the pubbococcogial (whoa, that's spelled really off!) muscles.