Tim Grey states that monitor calibration does not need to be done that often.
"Several photographers who have been learning about color management in my video course "Color Management for Photographers" have asked about how frequently they need to calibrate their displays.
The answer might surprise you. You actually don't need to re-calibrate your display very often! If you calibrate every six months, you'll generally have very good results.
This didn't used to be the case. With earlier CRT (cathode ray tube) display technology, there would be considerable shift in both the brightness and color balance for the display over time. Modern LCD displays are much more stable by comparison.
In particular, today's displays are very stable when it comes to color. The biggest issue relates to the luminance (brightness) of the display. Over time the display will get darker and darker. Fortunately, re-calibrating the display will enable you to compensate for that behavior.
But again, in general if you calibrate your display every six months, you'll be in good shape. If you keep a display for more than a few years, however, you might want to shift to a schedule of re-calibrating every three months."
I decided to ask my monitor manufacturer (NEC PA 272W) what they recommended.
Thank you for your request. Our SpectraView software, by default, is going to recommend that you calibrate every 2 weeks. This may not be necessary. Generally, you probably only need to calibrate once a month. It really comes down to your personal preference. Please let us know if you have any further questions."
NEC Display Service & Support