Piano technicians hear this question a lot. Before I became a tech myself I thought it was a little self-serving for my piano tuner to tell me that I needed to pay him a-not-insignificant amount of money to come back and do a job I thought should last for years and years. Boy, do I know better now!
Once one understands the immense amount of tension held in place inside the case of a piano, you begin to get a picture of what's going on “under the hood”. Think of other stringed instruments, a violin or a guitar, perhaps. The strings are held by pegs at one end, the go across a bridge, over a hollow point and then attach at the other end of the instrument. Think of how relatively small and thin those strings are compared to the strings stretched across the length of height of a piano. A violinist must re-tune his or her instrument Every. Time. They. Play. The same with a guitarist.
A piano technician has a multitude of techniques and tricks to help keep a piano in tune for as long as possible for the particular piano. And some pianos are inherently better at it than others. But still, moving your piano, shifting it further toward or away from a wall, leaving your humidifier on too long and many other things truly affect the tuning that your technician so lovingly applied.
All that being said leads me to say, if you play a lot at home, get your piano tuned every 6 months. If your piano gets played a medium to little amount, every year should do. If you're a serious musician, every month is a good idea.
