I do abs whenever I work out (cardio or weights), so anywhere from 4-6 times per week. I don't use weights when I'm doing abs - no ankle weights, medicine ball or dumbbells against the chest - so I figure it's OK to work them a lot because it's just my body weight I'm pulling up and down. The same with pushups - I'm still mastering "boy" pushups on my toes, and I'll try a set or two every day because I'm using body weight, not extra weights.