pH probes are one of the more neglected and misunderstood pieces of critical equipment in our tanks. To keep a probe lasting as long as possible, it is important to consider placing it downstream from filters and skimmers (cleanest water) and in an area of the sump/tank where particulates will settle and not clog up the reference port on the probe. It is also a good idea to keep the probe in as dark of a location as possible to avoid algae growth on the tip that will interfere with readings. Lastly, it is a good idea to clean your probe fairly frequently. Clogging up of the reference port, or tip, will significantly effect performance.
As for life span, there are several different types of pH probes based on whether they are liquid or gel filled and whether they are considered standard or stacked. Lab grade probes tend to be stacked gel filled which means they contain more silver chloride than the standard reference probes. The "œextra" silver chloride in lab grade probes is often a pellet that slowly dissolves adding more silver chloride to the tip which replaces what is lost through leaching. In either case, as the silver chloride leaches out of the tip, the probe will begin to fail. There's not much to be done about leaching, but when new either probe will be accurate. A lab grade probe will just last longer. To ensure your probe is still working properly, it is important to calibrate the probe frequently "“ monthly. However, at the beginning of the decline of a probe you will still be able to calibrate it, but you will know it's getting old by the time it takes to reach the tank's pH after calibration. There's no criterion for how long it should take, but if it takes 30 sec or so to drift to the tank's pH after calibration, it's probably getting too dirty or old. Keep in mind that even a dying probe will rapidly reach the calibration solution pH because of the solution's high ionic strength, so you can't judge by calibration time. The problem many reefers don't recognize is that even though the probe was readily calibrated in ph7 and either 4 or 10 solutions, it doesn't mean the probe is able to accurately read low ionic strength solutions (tank water) accurately.
In a good sump location, and cleaned frequently, a lab grade probe can last up to about 3 years. It is not a bad idea to replace the probe well before it fails. A neglected standard probe in a poor location in the sump or tank may last only 6 months. Also a probe used in very acidic or basic conditions will have a reduced life span, so check that probe in your calcium reactor more frequently "“ a probe that is slowly dying for a few weeks or months can suddenly fail without much warning.
The first approach is simply to wipe the probe with clean water and a soft cloth. Keep in mind the tip is glass and fragile, and there is a fine permeable membrane that allows the test article (tank water) to contact the reference solution within the probe (the reason they leach over time). Clog up that membrane and the probe is useless. Perhaps surprisingly, a common method for cleaning ph probes in the lab is to simply soak it in ph4 calibration solution for 30 minutes. I use this method and have a spare bottle of the calibration fluid for this purpose. I've had success bring back to life "dead" probes by soaking them in heated ph4 solution, but this may be too aggressive for routine cleaning. Vinegar will also work, but I think not as well as the ph4 solution.
Also, while perhaps counter-intuitive, it is best to rinse the probe between calibration solutions or after cleaning with regular tap water - not RO/DI water. Long term exposure to RO/DI (well more than the occasional rinsing) may cause premature probe failure.
While were at it, you also don't want to store the probe in ph4 or 10 solution. Use ph 7 solution.