Yup... Every... Day...
I am not condoning the total annihilation of your abdominals every day, mind you. My approach involves a strategy like this... Lower abs, then Upper Abs. I have found that since the lower abs are often slower to develop (at least for me, that is), I hit lower on Monday, upper on Tuesday, lower on Wednesday, etc. So, during the course of a week I work the lower abs three times and the upper abs twice (as long as I am doing body weight training). Obliques get worked on the same days as lower abs. Whenever I decide to overload the abs using something like a crunch machine, I totally skip abs the following day to allow them to properly recover.
🌟🌟🌟Ab training tips: When working the abs, go with minimal rest between sets and movements. Abdominal muscles recover quicker than most other muscle groups, so hammer them with constant movement! During each rep make sure to fully exhale instead of holding your breath. Your abs can fully contract only when your lungs are empty. If you are holding your breath, you are losing potential gains for your efforts. Work smarter, not harder!✌