Many pH probes will start to loose their calibration in as short as a month or two depending on your probe. We suggest calibrating probes every 30-60 days for the most reliable readings.
Almost all pH monitors and controllers will require two points of reference to accurately monitor the pH that the probe is reading. You will want to use calibration solutions that are on either spectrum of the pH you intend to monitor. For reef tanks where the pH is normally 7.8-8.5 then you would want to use calibration solution 7.0 and 10.01. For calcium reactors on saltwater tanks where the pH is normally below 7.0, you would want to use 4.01 and 7.01 for calibration of the probe.