The life cycle of ich is about 4 weeks. Which means that fish should remain in QT no less than that but really should be 5 or 6 weeks. As for the fallow tank, it should be at the minimum 6 weeks. Say a fish has the parasite and it lays it's eggs the day you take all the fish out of the tank. It takes 4 weeks for the eggs to incubate and hatch. Then, it takes a good 2 weeks for the ich to starve and die off. That is where the 6 weeks comes in but a little longer wouldn't hurt at all.