The short answer is twice a year. The longer answer is more useful, because how often a piano should be tuned depends on things a rule of thumb cannot see: the age of the instrument, the room it lives in, how hard it gets played, and what your ears expect from it.
Here is how we would walk you through the decision if we were standing at your piano.
Your piano drifts whether you play it or not
A piano goes out of tune sitting untouched in a quiet room. That surprises people, and it is the single most useful thing to understand about tuning.
The reason is the soundboard. Behind the strings of an upright, or beneath them in a grand, sits a wide panel of spruce, built with a slight upward curve that presses against the strings through the bridges. Spruce never stops behaving like wood. In damp months it takes on moisture and swells, the curve deepens, string tension rises slightly, and pitch creeps up. When the air dries out, the board relaxes, tension falls, and the piano settles below where it should be.
If the whole instrument moved together, you might barely notice. It does not. The middle of the piano, where the soundboard flexes most, moves further than the extremes. That is why a neglected piano rarely sounds uniformly low; it sounds sour, with octaves that no longer quite agree with one another.
Playing adds to the drift, and hard playing adds more. But humidity does most of the work, which is why the people who build pianos are unanimous on the subject. Steinway recommends three to four tunings a year. Yamaha and Kawai both call for at least two a year once a piano is past its first year. Nobody who makes pianos for a living suggests less. Once a year is not really a schedule; it is a floor.
What the Bay Area does to a piano
On paper our coastal climate looks gentle. In practice the Bay Area is a patchwork of microclimates, and pianos feel every one of them.
A piano in the Sunset or Daly City sits in cool, fog-fed damp for much of the year. The same instrument in Walnut Creek or Concord bakes through dry ninety-degree summers. And in every home from Berkeley to San Jose, winter heating pulls moisture out of the air for months at a stretch. The swing between those extremes, a furnace-dried January and a damp June, is what moves a soundboard, and the pitch with it.
You can soften the swing. Keep the piano on an inside wall, away from heater vents, fireplaces, and direct sun. If the room itself is changeable, a technician can install a small humidity-control system inside the piano that holds conditions steady around the soundboard. Clients who add one usually find their tunings hold noticeably better between visits.
New pianos are a special case
A new piano needs more attention, not less. Fresh strings stretch measurably through their first year or two, and every bit of that settling shows up as falling pitch. This is why manufacturers ask for so much service early on: Yamaha, Kawai, and Baldwin all recommend roughly four tunings in the first year, easing back to a normal schedule afterward. The same holds for a piano that has just been rebuilt or restrung.
Skipping those early visits does more than leave the piano sounding dull. It lets new wire settle unevenly, which makes real stability harder to reach later, when it matters.
The real cost of waiting
A tuning skipped is not a tuning saved.
A piano that misses a few years slides well below concert pitch, and at that point a single pass cannot fix it. Bringing the whole instrument back up means adding a great deal of tension across the frame, and as that load goes on, the soundboard compresses and the first strings tuned drift again before the last ones are finished. The remedy is what technicians call a pitch raise: a quick rough pass to get the tension close, then a fine tuning, and sometimes a follow-up visit once the instrument has settled. It costs more than the tunings that were skipped, and the piano spends the years in between sounding worse than it should.
There is a quieter cost too. Anyone learning on that piano is training their ear against it, every single day. If weekly lessons are involved, an instrument that has drifted far from pitch works against every one of them.
A simple way to decide
For most pianos in most Bay Area homes, twice a year is the honest answer, and it lines up neatly with the two humidity swings the instrument actually lives through. From there, adjust for your situation. A piano played daily by a serious student or a teacher earns three or four visits. A lightly played piano that has proven it holds well might manage on one visit a year, though it should never go longer than that. A new or newly rebuilt instrument needs its first-year tunings no matter what else is true.
The best guide, though, is not a chart. It is a technician who has tuned your piano more than once. After a couple of visits they know exactly how far your instrument moved since the last one, and they can tell you what schedule it actually holds on rather than what a rule of thumb suggests. That is one of the quiet advantages of having the same person return to the same bench.
Put it on a schedule and stop thinking about it
Most of our clients settle the question once: they put the piano on a recurring schedule and let us keep track of it from there. When you request service, a real person reads your note, asks where you are, and connects you with one of the regional technicians serving the SF Bay Area, someone who covers your area and can get to know your instrument over time.
And if you cannot remember the last time your piano was tuned, you already have your answer.