Mine sat for 3 and a half years, with water in it. first time I made water for the restart, the numbers stayed up in the 20ppm area for about 2 gallons, just as I was about to turn it off and replace DI media(s).. it dropped to 2-3, then about 1/2 gallon later, 0-1. Have since made 100 gallons or so, all at 0-2..