Dipping your fish is a tricky situation. Even if you dip your fish, it's not 100% that you will cure the fish. You may actually get rid of the disease, but you may stress out the fish so much that it will get it again.
From what I remember, a 30 to 60 second dip should be ample time for treatment.
Instead of dipping, consider treating a fish in a chemically-treated hospital tank.