my abs get trained during every lift session, so usually 3-4 times a week. just about every compound movement i do involves the abs very nicely. (squats, deads, chinups, good mornings, etc)
very occassionally, i'll throw in planks, hanging leg lifts and standing weighted crunches. but that's only ever on wed evenings, when the 6:00pm step class is going on and i need a reason to be standing there watching.