I personally would treat until I saw a signifigant(sp.) change in the "hole". It also depends on why he has the whole in the head disease. Is it because of poor water quality/stressful conditions or because of the parasite that has also been know to cause the issue. If it is because of the water issues doing several water changes a week until improvement will help him out, if its because of stress ie. too small a tank, too many tank mates etc. you should change his situation, and if its because of the parasite I would treat with the propper anti-parasite medication until you saw improvement.