The life cycle for ick is right around 3 weeks, thus go with a MIN. of 3 weeks. This does not mean to dump cooper in your tank everyday for 3 weeks, it means that you need to maintain theraputic copper levels in your tank for a min of three weeks [however, I would go with a whole month just make sure]. Also this applies to maintaining the copper for 3 weeks after the LAST sign of ick on your fish. Thus if your fish has ick on for a week, then you will treat with copper for a min of 4 weeks.
Also, to maintain theraputic levels of copper, you use a test kit that will measure copper. DON'T keep adding copper everyday or every other day. Use the test kit to get an accurate measure. Copper is poisonous to fish as well as the parasite. The goal is to use enough copper to kill the parasite, but not enough to kill your fish.