If the fish is by itself, the other option, which is my first choice in meds for this sort of thing is a mixture of methylene blue and fungus eliminator. The trick is to avoid 2ndary infections from setting in. Honestly, this is easiest done if the fish is in a bowl alone. If you need, I can post the best med procedure done the bowl method.
As for how long... that depends on water quality, temp, and the genetics of the fish. I haven't had as much luck with the melafix as the other method, but not all fish can tolerate the methylene blue. If there are other kinds of fish in the tank, melafix is the safest way to go.