Well typical with rain rot you know it's gone when there are no more scabs forming and the hair starts to grow back. I usually continue treating for a few days after I notice no more scabs just to be safe. But some horses it takes a long time. I've been treating my gelding for about 2 months now and we are making some slow progress. He's healed about 76% of the way. But some horses are just more prone than others.