That depends on a few factors. How much is it being used? If it is being played a lot, say anything from 3 or 4 times a week to a few hours every day, It is best for the piano to be tuned at least twice a year. In regions where the seasons produce dramatic changes in temperature and humidity, and when the piano is heavily used, a piano may need to be tuned every 3 months.
At the minimum a piano should be tuned once a year. When a piano goes out of tune the tension in the entire support structure changes, and if this is not corrected it can lead to instability in the instrument and loss of market value.
My piano has not been tuned in a long time. What now?
Of course there are times when the faithful family piano falls out of use for a while. The kids grow up, Aunt Betty who used to come and play on family visits moved away, the years slip by before you know it. But then one day you want to resurrect your piano; the holidays are coming up, or new kids in the family are learning.
A piano technician can come out and look at your piano, and tell you what kind of state it’s in. Sometimes, if the piano has been in a protected environment and the lapsed time is not too long, a simple tuning will be sufficient.
Most of the time, though, if more than a year or two has gone by, the piano will need what is known as a “pitch raise”. This requires at least two passes at the piano and for that reason generally costs extra.
What is a pitch raise?
When a piano has not been tuned for a while, the overall tension on the piano will deteriorate. Pianos are designed and built to support a certain amount of tension, that needed for the piano to be tuned to the international standard pitch of A at 440Hz.
When the overall tension on a piano drops, the pitch over the whole keyboard drops and before the piano tuner can do a fine tuning, the pitch over the entire keyboard has to be restored to something close to what it is designed for. The piano tuner will do a first pass, pulling up the tension over the whole keyboard in a way that accommodates the changes in tension that occur while adjusting other strings. Then the second pass is the actual tuning.