I filled a old wash tub with 6 gallons of EVAPORUST and dipped many parts in it and it lasted about 4 months. You will know when it quits working as it will not work at all. When mine was used up I drained 5 1/2 gallons back into a container so evaporation was not a problem even in the warmer months. I just had a cover over the tub and it was not air tight. I have found the product works better in warmer temps.