When I first learned about screen calibration from Zack Arias, he mentioned that it should be done at least once a month since screen colors change over time. I didn't really believe it would be something noticeable. I thought calibrating the screen only one time would be good for at least a year.
A long time ago, when I first bought a new 23" Samsung LED monitor (upgrading from a perfectly fine 19" LCD), and discovered that all the colors in my pictures looked wrong. It was devastating, and no matter what calibration guides I followed, I wasn't able to get correct looking colors. So I bought an X-Rite ColorMunki Display and finally had a calibrated monitor, with correct looking colors.
Back to the present, I now have a better monitor that was last calibrated 6 months earlier. A couple of days ago, I decided to re-calibrate the screen and see if anything has changed (the software shows you before & after). To my shock, I discovered that my screen colors has gotten very warm compared to after it was calibrated, it looks like something close to 600 Kelvins of warmth! That is a significant difference, and it will make go back and edit several of my recently edited photos that I intended to give a warm look, but now looks cooler/bluer after calibration.