This is a common question in the piano world, and the answer varies (depending on who is asking).
If you just bought a new or pre-owned piano, it should be tuned at least 4 times in the first year. The piano is adjusting to it’s new climate and will need the extra tunings to stay at A440. Once you’ve had your piano for over a year, we recommend tuning no less than twice per year. Some people will choose to do it more often such as piano teachers and concert performers, as there demands of the piano are higher than average. Many piano dealers offer free tunings in the first year of ownership, make sure you take advantage of this!
If it’s been awhile since your piano was tuned, it my require a pitch raise or lowering before the fine tuning can take place. In this case, the piano will need to be tuned again just 30 days after the pitch raise appointment. Usually, after the 30-day follow up tuning you can resume tuning every 6 months. In some extreme cases the piano may need to be tuned multiple times after a pitch raise. There is an earlier blog post dedicated to pitch raises which you can check out here.
If your piano is mostly used as a furniture piece, it is still important that it be tuned at a minimum of once per year to ensure that the piano doesn’t fall into a state of disrepair and lose it’s value. If a piano is allowed to sit for long periods of time, it will still go out of tune, even without anyone playing it! If you are planning on passing your piano down to your children or relatives, you don’t want to leave them with a piano that will require costly repairs that could’ve been avoided. Worse yet, if the piano has sentimental value, it’s even more important that it doesn’t fall into a state of disrepair. Choosing between costly repairs or junking a family heirloom is a difficult decision.
Lastly, some pianos are tuned once a week or even more often! Pianos in recording studios or university environments receive lots of attention and lots of care so that they play and sound as good as possible for the most discerning performers and listeners. There’s a reason why concert pianos sound so good when you hear them in person at a concert hall or on a television broadcast, they are meticulously maintained be the best technicians in the world.