Met quite a few successful lifters (BB'er & PL'ers) who hit a selected muscle group, or lift, every 7 to 9 days. Anything longer than that, most tend to not keep, even slightly losing, their training/conditioning edge. Usually the 7-9 thing is when extra heavy workouts are done. I'll do once a week usually. Think most trainee's would find once or twice a week to their benefit, when working a targeted body part. Three times a week, to me, is just not needed. Even full body workouts (becoming somewhat popular now) can be preformed once or twice a week.
As dj181 suggest, direct ab training may be over hyped. The Ab's are a very powerful short range muscle group (as most short range muscle groups are) and should to trained that way. Regular BB'ing rep of 8 to 12, 2 or 3 sets should be all that is need. Even indirect exercises, like chins, pullovers, cleans,dips, etc, hit the total Ab wall (obliques also). If finding happiness in marathon ab workouts, than help yourself. Fear not of adding impressive weight to any of your Ab workouts, they are designed for strength.
One of the better Ab training devices is the common, and much over looked, Ab Wheel. Another device it the original Roman Chair. Extremely effected. They both hit the total Ab structure, upper and lower, with equal attention. Some of the exercise equipment that are being sold as Roman Chairs, in reality, are far from the original concept and not real Roman Chairs. Good Luck.