One of the most frequent questions Aaron Marino of alpha m. receives is about abs-- how many times a week can you hit abs? The answer varies.
Aaron only hits them once a week-- the rest of the time, he does cardio and eats well to make them show. If you're doing a few sets of crunches, you can do them every day. Aaron demonstrates his workout which includes the warm-up (basic crunches), cables (standing and floor), bench weighted crunches, and leg lifts. He'll do 5-sets of that. If you feel any pain or discomfort in your lower back, stop and modify.
If you work them like this, you only have to do abs once a week. Combine with cardio and eating well, that's how you get the abs to show.