2-3 times a week for me as well. I took a class on this (not an exercise class, an actual educational class) about 10 years ago that debunked many myths about ab training. I was doing the old 300 crunches every day thing & the class taught exactly what Mr. Horton said--abs should be worked like any other muscle, including using a weighted routine. If you want a six pack you have to build the muscles, & if you overtrain a muscle you shrink it.
Another interesting thing I learned from the class was that ab exercises where you use your legs as a lever (i.e. the roman chair) work the hip flexors as the primary muscle & the abs as the secondary muscle. Exercises like corkscrews & such--when you're "pushing" upwards rather than "swinging" up & down--use the abs as a primary muscle, but exercises like leg lifts use the hip flexors as the primary muscle.
Finally, I suggest you do a search on the Ask Cathe forum. She's given out a lot of great information in the past about her own ab routine & options for ab work.