I treated one of my Bettas with the Ich killer stuff by adding some everyday and leaving the light on for a month. You have to kill off the eggs that are in the water too, so they will not attach to the fish. I treated the fish for a month with the medicine and I do not think I even did a water change or maybe I did once during that time--Can't remember. But it was definitely a month long treatment.
Hope your fish gets well soon!!!
Hope your fish gets well soon!!!