The short answer: At least once a year.
And now, the details: For many people, getting your piano tuned once a year will meet your needs and will keep your piano in reasonably good tune throughout the year. There are some situations, however, when you might want to consider more frequent tunings.
For example, a new piano, or a newly rebuilt piano, or any piano that has new strings, should be tuned more frequently than once a year. For the first year with the new strings, 4 tunings will probably be in order. The reason for the additional tunings is that new strings take some time to stretch into a stable position in the piano. Piano strings have a number of bends and contact points: around the tuning pin, at the agraffe or v-bar (on grand pianos) or plate nut or plate pin (on upright pianos), and at the bridge pins.
Piano strings take some time to adapt to the high tension and new bends that are created when the strings are new. As a result, new strings always go out of tune rather quickly after they are tuned. It may be a matter of hours, days, or weeks, but the new strings are going to go flat as they stretch and react to the changes. After the first year with new strings, you will likely be able to get your piano tuned once or twice each year.
Another reason for more frequent tunings is for pianos that are played more. Generally it's a myth that pianos need to be played in order to stay in tune. Actually, playing a piano tends to gradually knock strings out of tune. So, pianos that are played a lot tend to go out of tune more quickly than pianos that are played less.
Seasonal humidity fluctuations can have a large effect on a piano's tuning. Pianos tend to go sharp (going higher in pitch) in the higher humidity months of the summer, and go flat (going lower in pitch) in the drier winter months. And the pitch changes in a piano are not uniform: the changes occur most in the middle range of your piano, while the bass and treble ends change less. As a result, seasonal humidity changes can put your piano out of tune with itself, because the middle of the piano will either be sharp (in summer) or flat (in winter) relative to the bass and treble sections. You might want to consider having your piano tuned more than once a year if the seasonal humidity changes are causing your piano to go out of tune. You might also want to consider getting a humidity control system installed in your piano.
Some piano owners prefer to get their pianos tuned twice a year, at times of the year that correspond with the seasonal humidity changes. Generally these two times would be in the early- to mid-range times of the humidity changes, such as perhaps May and November, or June and December, or June and January. If it's important for you to keep your piano at a specific pitch, such as 440 Hertz (440 cycles per second) for the A above middle C, then we can certainly keep you piano at that pitch whenever possible when tuning. Other piano owners might prefer to adopt the "floating pitch" approach, in which the tuner might work with the piano's natural inclinations to tune it to, for example, 441 Hertz in the high humidity months, and 439 Hertz in the low humidity periods.
Finally, you might have a sensitive ear, and you might be bothered by certain aspects of your piano's sound, even if the piano is only slightly out of tune. Consider that pianos on concert stages are tuned before every performance, and some even receive touch-up tunings during the intermission at a concert. It's a fine idea to get your piano tuned multiple times a year to keep your piano sounding in a way that makes you happy and allows your own artistry to shine.
The most important consideration is what your own needs and desires are. It's best to get your piano tuned at least once a year. And if you like to have your piano tuned more often, for any of the reasons described above or for any reason at all, that's great too.