  Store bought Clorox is 0% sodium hypochlorite when it leaves the factory but slowly breaks down over time. If you are buying bleach do it in a store that has a high turnover rate to insure you are getting the most strength possible. I used to buy industrial bleach (00%) by the 00 gallon barrel from a chemical supply house that was very good about keeping fresh product on hand.  A quick test to see if your bleach has any punch is to dip two fingers into it and rub them together. If it feels like it is getting warm then it still has some zing to it.  