How long does this product work? Will the mold or mildew come back? How often do have to reapply?

Wet & Forget works over time with the rain and wind to gently clean up moss, mold, mildew and algae. If you treated a surface with green stains, you should see results 1-2 weeks after applying. If the surface has black stains, you should start to see results 1-2 months after treatment. Wet & Forget is not designed as a preventative, however since Wet & Forget cleans the surface properly it can keep growth away for 12-18 months. We recommend applying once a year as a maintenance tool.