I'd calibrate it on a regular basis, such as once a week. Some people with color critical work do it every morning. It all depends on what kind of work you do and your results. If you do notice a difference, then you probably need to do it more often, as you should be keeping the machine producing at the same levels.
If you don't have a humidity controlled environment, you will probably need to do it more often. Humid conditions can throw it off. There is also an argument over whether one calibration can be enough for all the calibration sets, as is provided for in Command Work Station, or whether you have to calibrate for each set. My last tech said one calibration for all is enough, but others have been told to calibrate for each set. You might ask your tech.