Lt. Bob said:
Those are the key issues. At least once a year every single year. All the reasons that people give to not tune their piano ( "oh, no ones playing it" or "It still sounds fine") are not valid. Since a piano goes out of tune over a years time.......you get used to it and it sounds ok.
Also, it can take a few tunings (i.e. a couple of years) to get a piano fully stabilized and it is helpful, as Twist mentioned, for the same tuner to do them because he will get a "feel" for that particular piano. Every single one of the damn things is different and behaves differently. I usually find once a year is sufficient for most people. But that might not be enough in places where the temperature changes more with the seasons than it does here.
Lastly, if the piano is below pitch very much it can take multiple tunings to get it up to pitch and stabilized. I consider anything over 1/4 step flat to require multiple tunings. The more below pitch, the more tunings. 1/2 step requires three tunings.