Phil Eisermann [mazer at ix.netcom.com] says
Of course, if you have ever left your pH probe in your saltwater tank for any length of time, you'd find it was impossible to recalibrate. I know this from personal experience. They tend to last around 8 months or so. I have never had one last more than one year.Keep in mind this will vary with the quality of the probe. I finally tired of replacing the $50 PinPoint probe twice a year. I store my probe in an "electrode storage solution" from Hach.
See also:
http://www.electrodesdirect.com Factory-Direct Supplier of pH, ORP, Conductivity, Dissolved Oxygen, Ion Selective Electrodes, Meters, and Monitors for the laboratory, process, and biotechnology industries.