The two questions piano tuners are asked most often are "Why does my piano go out of tune?" and "How often do I need to tune my piano?"
Why does a Piano go out of tune?
There are two main reasons why a piano goes out of tune. They are climatic changes and usage.
Your piano is mostly made out of wood and metal. Both of these materials are affected by changes in weather. Timber is affected by changes in humidity and metal is affected by changes in temperature. When the relative humidity rises, timber absorbs moisture from the air. This makes the timber swell. When the soundboard in your piano tries to expand it puts more pressure on the strings and the pitch will rise. The opposite happens when the air dries out- the soundboard shrinks, reducing pressure on the strings and the pitch drops.
Steel is affected by changes in temperature. When it gets hotter it expands, and when it cools it shrinks. The strings in your piano have little mass and will change relatively quickly. When they get warmer and expand the strings will loosen and the pitch drops. When strings cool they shrink and tighten and the pitch rises. The frame of your piano is also affected by temperature changes, but the extra mass in the frame means it changes much slower than the strings.
Now think about how much temperature and humidity change during a single day, let alone in a month, season or year.
Every time you play a note on your piano you cause the hammer to strike the strings. The more often you do this, and the harder you strike the strings the more out of tune your piano will become. It's similar to servicing your car. The more kilometres you do in a year, or the harder you drive, the more often you need to change the oil.
If you have invested your money in a good quality instrument, it would be wise to protect your investment with proper care.
How often do I need to tune my Piano?
If you think about why a piano goes out of tune, you'll realise that if you wanted to you could tune a piano every week! In fact, for concert and recording venues where an extremely high quality tuning is expected, that is the minimum required. For most domestic pianos once or twice a year is enough. A general rule is that beginners need a piano tuned once a year, but as skills, techniques and workloads increase, so should the frequency of tuning. By the time someone is doing AMUS level the piano may be needing tuning 2 to 4 times a year.
Every piano should be tuned at least once a year, regardless of how much it's used. Most piano manufacturers including Steinway and Yamaha recommend two tunings per year.