Depends on your goals and what other exercises you're doing. I think a whole day is a bit much for most people. The most I'd ever want to see in one routine is one abdominal, one oblique, and one torsion exercise. (for example crunches, side bends, and twists) and honestly even that's probably overkill.
I usually add one of those exercises to my routine, and rotate every few months or so. Your midsection should be getting hit pretty hard from doing compound freeweight exercises. At little extra isolation work isn't bad, but no reason to get all crazy about it. This advice may not apply if you're training for a sport.