How often should a piano be tuned?

The short answer:

•  EVERY SIX MONTHS

     
The long answer:
It really depends on many things. Most of all it depends on your own needs for your piano. Concert pianos might be tuned several times a week. Manufacturers' recommendations vary from 2-4 times a year. The things that put a piano out of tune are:

• CHANGE IN RELATIVE HUMIDITY   See Humidity Control   A piano starts to go out of tune as soon as a tuning is finished. Well, it has no where else to go. How much it changes depends on how stable its environment is.
  
• MOVING   A piano cabinet seems very hard and rigid. It has to be, but when it is picked up, the case will flex slightly and the tuning will change slightly. Moving in a truck can cause the piano to bump and vibrate. This upsets the tuning too.
• WEAKNESS IN THE STRUCTURE   This is a big one. The soundboard, the tuning pins, the the friction points along the string as well as the cabinet itself can and do all change, which changes the tuning. Some pianos have design weaknesses in one or more of these areas. Some have developed weaknesses just due to time and their environment. In the ideal piano nothing would ever move even the smallest amount, but such a piano would not sound very good.
• ENVIRONMENT   Anything that causes uncontrolled changes in the room affects the piano. Open windows, forced air registers, fireplaces, fish tanks, plants and freshly shampooed carpets all have very harmful effects.

Ask yourself how fussy you are and what you can afford. The best tuning results come when the change is the smallest.