Generally speaking, your piano should be tuned at least twice per year. When the piano is tuned on a regular basis, good things happen across the board. First, since the instrument did not drop way out of tune, the piano tuner can really fine tune the instrument now, resulting in a more accurate tuning.
Take a moment to picture an auto mechanic performing a tuneup at a service station. The first car is running so rough that the mechanic replaces all the spark plugs, the air cleaner and the fuel line filter. He also changes the oil, refills the coolant and checks all the spark plug wires. Now the car runs properly. The second car he works on was tuned up six months ago and runs almost perfectly. Now the mechanic can take an hour to really fine tune the carborater and check other systems under the hood. In the end, the car is really fine-tuned now.
At the same rate, the piano tuner runs into the same type of job, so to speak. One customer has a piano that has not been tuned for over eight years. Now the tuner must do a pitch raising, stretching all the strings above the pitch that they will be tuned to, then tuning them down to their respective pitches. Chances are a few keys are sticking and need adjustment or even replacement. Now the parts may be replaced immediately if the tuner has them on hand or may have to be ordered. Now the tuner will return within a couple of days or weeks to replace the part. In the interim, the piano may have some notes that do not play right. This first piano job is more expensive and took longer to complete.
The second piano the tuner works on was just tuned six months ago. Although the piano is out of tune, all keys work, the pedals are working fine and all systems are in good working order. Now the piano tuner can focus on performing a regular tuning. The strings do not need to be stretched, nothing needs to be fixed and no parts need to be ordered. As a result, the tuner will spend a good hour or more really fine-tuning your piano so it sounds even better then the last tuning.
That's the goal of the piano tuner: to make your piano sound even better than the last tuning - every time.
Don't let your piano slip way out of tune because it costs everyone more time and money to get it back in shape. Instead, have your piano tuned every six months. This will insure that everyone who plays the piano will enjoy what they are playing and will not be listening to the wrong information; the wrong notes. It is very important that students are only exposed to instruments that are in good tuning order. This will help them develop their listening skills more easily.
It's well worth the money to have your piano tuned every six months.