A lot of posts in this thread, but I wanted to try to answer this question.
Lets say you have a qt that is now infected with ich. There might be some on fish, some in the water, and some encysted on the walls of the tank.
You add copper to the water which will kill ich in water, but not necessarily kill the ich stage living on fish (it's under the fishes' skin) or the ich stage encysted on the hard surfaces in the tank. Copper is used for 14 days.
Now, the ich in the water is dead. As soon as the feeding stage of ich falls off the fish, it will die. This will take less than 14 days (3-7 days according to Humblefish), so you are covered there.
Now you are just left to worry about the encysted stage, which can last (maybe) as long as 72 days. Any ich that becomes free-swimming to look for a fish in the 14 days will die in the copper-laced water. Any that stays encysted longer than 14 days is of no consequence, since you will move the fish to a different (sterile) tank after 14 days.
Hope that helps.