One will last about 3-4 years before the foam starts to go bad.. and/or about 50 cleanings using what the oil manufacturer recommends..not what people say you can get away with. Just spray or put on..and inside enough that you can work it all the way through and coat every cell, both layers. Then grab 2-3 paper towels and stuff them inside..give it a light wringing twist to grab and excess that could be pulled in. Then install the outer layer and let dry for a while. I set mine in the sun for an hour. Then install. Keep the ones you are not using in a zip-loc bag in a cool, dark place. You can pre-treat them and they will stay good and sticky for several months.