A lot of people seem to think the recovery period with abs is virtually non-existant, training them everyday. I've also heard it said that abs should be overtrained to create smaller tighter muscles. I like to treat abs just like any other muscle. I hit them hard and pretty heavy twice a week, and they seem to respond pretty good to that. But, I know people that train them everyday for 20-30 minutes and their abs would break my knuckles. Go figure
Once a week, especially since abs are involved in pretty much any excercise you do. But realistically, when most people ask "how do I train my abs?", they should be asking, "how should I be eating?"