My son purchases this item after trying several things to get the terrible rotten egg smell out of his water. He said it took about a week but the smell disappeared completely. I purchased one afterwards and found the smell was gone almost immediately. My sister purchased one for her new home and it started working the same night. She now enjoys her showers again. This is my second one for another smaller heater in another part of my house. This thing is a life changer. Don't be afraid to purchase.