I do not guarantee a specific time for moss not to return. Companies that do that are crazy, usually the start ups do that because they don’t know. I have been doing this for 10 years and I still have no crystal ball. I would say that if you treat your roof every year you should be good, some houses take 2 a year but that’s rare. The key is to keep an eye on your roof and stay ahead of the problem. If you see green, let me know and I will treat it before it turns to moss.