I think the normal time length for treatment of ich is 15 days, but may vary depending on your medication you are using. Here is a quick rundown of the life cycle of the parasite.
The visable stages of Ich are carried out within the host fishes' skin. The first stages are called trophozoites and are highly resistant to drug therapy. Trophozoites mature into trophonts and leave the host, falling to the bottom of the aquarium or pond.These mature trophonts release from 200 to 1,000 tomites. These tomites move about looking for a host, which they must find within 2 to 3 days at 75° F (24° C) or they will die. Cooler temperatures will lengthen this time. It is this free swimming stage that is most vulnerable to treatment. It is important to note that these intermediate stages may also attach themselves to plants and be accidentally introduced into an aquaruim or pond along with the new plants. Once the tomite attaches to the host, it matures and the cycle begins anew.
Just keep up the dosage and in a few weeks they will be back to normal.