Then, as you feel they are getting less sore and better conditioned for that type of workout, you should add a few more sets to each session or throw in new exercises, like hanging knee raises.
I take the 2-3 times per week approach (when
I started training my
abs,
I began with 1 session a week and worked my way up). I think this is much better because you are still training the muscle often enough to get great results, but you're not over-training the muscle.