In general lab practice calibration (
http://www.ph-meter.info/pH-electrode-calibration) should be performed before every set of measurements, or at the beginning of every workday. However, in the case of aquarium, when the electrode is measuring pH continuously, it can be done much less often.
Note, that it will depend on the electrode make, thus it is difficult to give general, bulletproof advice. If you have seen suggestions to calibrate electrode once per month start this way, just keep track of the results - write down pH indicated by the electrode every time it is taken out from the tank and put into pH 7.00 buffer. This difference will be probably greater with time - at some point you should decide to repeat calibration procedure more often.
When the electrode is completely new, calibrate it more often - say daily in the first week, then every week in the first month. That will give you good point of reference, you may treat these first results as "ideal" electrode, thus later you will be able to say whether electrode deteriorates just comparing initial indications of the electrode put into buffer with these of the fresh electrode in the same situation. It will also help to identify faulty electrode (rare case, as they are factory tested, nonetheless it may happen).
Remember to calibrate the electrode always in the same temperature water in the tank have. The less variables changing the better.
