At LEAST once a month, pH Meters especially. The cleaner you keep the Probe of your meter, the better it will work. Letting film or salt buildup on the tips of the Probe can be just as bad as letting the tips of the pens dry out. The tip of the pH probe is porous to allow the solution to be absorbed. If you look at the tip of almost all pH meters you will see that there is liquid in the tip, if the tip dries out the meter will not work. You can use distilled water, Calibration solution or Storage solution to keep the tip moist. Put a piece paper towel in the cap of your meter and pour a little solution in it. This will help stop the probe from drying out.