When I got my mare she was kind of a rescue that came to the barn where I board. You could count her ribs, halter marks were dug into her face, and she was completely covered in rain rot. I can't remember how long it took to clear it up now (it's been almost 9 years ago now, wow time flies) but I'm thinking it wasn't more than a couple months and it was severe rain rot. All I used was EQyss' line of sprays and shampoos and brushed and brushed like crazy. And she's never had an issue with it again, luckily. Good luck!