It is an ongoing process.
Some people calibrate once and forget about it.
Some people start calibrating and cease to photograph due to a near obsession with film densities. A good route would seem to be somewhere in the middle. Any calibration is better than no calibration. Some calibration isn't as good as enough calibration. Of course as you add lenses or new films to your camera bag, or change enlargers or change primary papers for printing, you will create opportunities to recalibrate your system.