Well, I work my abs usually 5 to 6 days a week. I got into this habit while doing Slim Series workouts. Debbie works the abs with each workout. A lot! I am pretty happy with the results!
I think you can work your abs/core every day, so long as you don't work it the same way every day. For example, traditional abs work like crunches one day, plank work another day, stability ball another day.