Total newb here, but I would plan to maintain 10 ppm FC for several days, or as long as needed until your chorine demand is back in check. There is a Chlorine Demand sticky in the resources tab, so give that a read if you haven't already. The trick here is to maintain shock levels until you are back at baseline contamination levels. It could take several days and you will need to test and replenish FC frequently at first. Good luck. Vigilance is key.