You only need to shock if you let your chlorine level drop below the range that you are supposed to keep it in based on your CYA level (see Best Guess chart below in my signature), or if you have a really big swimmer load, or if you have a CC reading above 0.5. I also sometimes shock if I have a big storm that throws a lot of debris in my pool. Other than that, you don't really need to shock. I rarely shock my pool because I maintain my chemistry where it needs to be. I'm guessing other than at start-up, I probably don't shock my pool more than twice a summer and maybe not even twice. Of course pool stores tell you to shock weekly. They want your money!