My last set lasted 9 months appx with the ph
going first. It really depends on how long it sat on the shelf and where you store it at your house I guess. I noticed it had gone bad because the ph
test went from 8.2 one week to 7.6 the next with no noticable diff in the tank. So I tested the other two tanks and the same thing so I knew it was the test kit and got a new one, and voila! it was back to 8.2 again. I would start to look for weird readings between 9 months and a year. But look for an expiration date on the box if available.