Having a piano tuned is the most common maintenance task for owners, yet many lack basic information about this necessity, such as the reasons a piano goes out of tune or how often a tuning should be performed.
Why pianos go out of tune
Humidity changes are the primary reason that a piano loses its tune. Wood is very susceptible to moisture and swells and shrinks with the changes in the air's humidity levels. The piano's soundboard, being a large wooden membrane, swells when it takes in moisture during times of high humidity. This puts extra tension on the strings via the bridge, causing them to rise in pitch. The reverse is true when humidity levels are low, the soundboard shrinks and tension on the strings is reduced, thereby lowering the pitch. It's important to remember that this phenomenon occurs with all pianos, regardless of quality, and is something that cannot be changed.
The part of the soundboard that undergoes the greatest amount of bulging and constracting is in the center, where the low treble and tenor ranges are to be found. So, it is the middle range of the keyboard that tends to go out of tune the most. This fact often leads one to conclude that the act of playing the piano (most of which is done in the middle register) is the primary cause for the loss of tune. Moderate to heavy playing can speed the process of tuning degradation, but it is not the primary cause.
Another misconception is that moving a piano causes it to go out of tune. This is only the case with the cheapest instruments and is caused more by unsound construction than any other factor. Moving a well-built piano should not destabilize the tuning unless it has undergone a long journey with rough handling, in which case it may be that structural damage has occured and caused the tuning instability. In fact, the change in tuning that often occurs when moving a piano from one geographic area to another is due to the humidity differences rather than the physical process of moving it. When moving a piano around a room or within the same geographical area there should not be a noticeable change in it's tuning.
How often and when you should tune a piano
Ideally, pianos should be tuned during periods when the humidity level is stable. In other words, they should not be tuned during seasonal changes when humidity is changing rapidly. Different geographic climates have varying extremes of humidity, so a local piano tuner should be able to advise on the best times to tune in your area. Typically, humidity levels begin to rise in mid-spring and drop in mid-autumn, with the dead of winter and summer offering the longest periods of relatively consistent humidity levels. However, since pianos are kept indoors it is the indoor humidity that is our main concern. So, the indoor humidity changes occur when the heating unit is turned on an off. Therefore, it is best to time a piano tuning a few weeks after the heat is turned on or off.
Tuning at least once a year should be the minimum for anyone who values their piano. Furthermore, this tuning should be done around the same time each year to avoid large pitch adjustments. During the year's seasonal changes the piano will most certainly rise and fall along with the humidity changes. But, by the time the annual tuning approaches its pitch should have settled back to a range very close to that during the last year's tuning.
Most tuners recommend two tunings per year to keep the piano's acoustics at a level that is tolerable to the listener's ear. Ideally, this would be done as stated above - shortly after turning the indoor heating on and off. There is one drawback to the bi-annual tuning schedule though. Because one tuning will be done in late spring/early summer, which is when the humidity levels are highest, the soundboard swelling will have increased the pitch of the strings to their sharpest level. A similar, but opposite, occurrence will take place in the winter when the pitch of the strings are at their flatest level. This means that large pitch adjustments will have to be performed to tune the piano. These large pitch adjustments are notoriously unstable and will usually require follow-up 'fine tunings' and more expense. Unfortunately, there is no way around this problem except to tune the piano more often.
The next, and best, recommendation for tuning frequency would be four time per year. Naturally, this will cost more and may be financially unfeasible for many. However, it is really the only solution for the bi-annual tuning problem stated above. It is also the best way to keep the instrument within a range that is tolerable to listen to year round. The recommended schedule would be the same as above with two added tunings in the interim.
In the final analysis, the frequency of your piano tuning is really going to depend mostly on your budgetary constraints and personal tolerance for listening to an untuned piano. There really isn't any danger to the piano if it goes untuned for a number of years. However, one shouldn't leave a piano unplayed or unserviced for such a long stretch of time, as playing it often can alert one to mechanical problems that may be developing.
For those that are DIYs, learn more about tuning a piano yourself.