How Often Should I Tune My Piano?
Well, that depends.
If the piano has been well maintained the basic rule-of-thumb is at least once a year. Twice a year would be better. There are a number of factors that can contribute to the piano going out of tune, even if it isn’t played very often. Consider that the piano is primarily made of wood, and that at it’s simplest level the sound is produced by striking the strings with a hammer (no, not a carpenters hammer, the felt covered hammer that moves toward the strings when you press the keys). Changes in humidity can cause the wood to swell (too much moisture) and shrink (too little moisture). The combination of swelling and shrinking goes a long way toward knocking the piano out of tune, and can cause other issues as well.
Changes in temperature can also effect the tuning stability, but generally not as much as the changes in humidity. In fact many of the high end piano manufacturers write into their warranties that you must maintain the humidity in the pianos environment within certain parameters (more on options for accomplishing this later) or you could void your warranty.
So call a qualified piano tuner-technician to service your instrument regularly, and pay attention if he/she recommends taking steps to balance the climate in and around the piano. The piano has thousands of finely regulated moving parts and yet is designed to support literally tons of tension from those roughly 230 strings. Take good care of it and you should experience many years of enjoyment.