You need to keep adding the chlorine for the whole time the pool is up....but you need to keep it at shock level until the green is completely gone and you are not losing any chlorine overnight (testing in the evening and again in the morning before the sun hits the pool). At that point you can let it come back down to normal levels, but you really need to know if you have a CYA reading because that's what your chlorine levels are based on (see the "best guess chlorine chart" link in Pooldoc's sig).