pH tests are fine for years. Our club did an experiment once. We got an expensive, calibrated pH test meter then asked members to bring containers of tank water, tap water and any pH test kits they had. We then tested all to see what range of pH we had in our tanks and whether the differing test kits gave different results.
Interestingly enough, all the kits read the same and matched the expensive electronic meter. Some of the tests kits were brand new and some were many years old.