Piano Tuning and how often it should be tuned is a somewhat tricky question to answer. There are many factors in play, for instance, let's assume it is a church or school piano. These instruments are often subject to radical  humidity changes. In the case of a church, people usually show up in church on Sundays and Wednesday nights. When there are many people in Church, the humidity rises. Our bodies are made up of a high percentage of water and we affect the tuning of the piano. We then leave the sanctuary and the humidity drops. It does not take to many times of this and, you guessed it, the piano is out of tune. Schools are the same way only can be even worse. These pianos should be tuned quarterly.

Then there is the case of the home piano. Are there young children that are taking lessons? For these pianos, special care must be taken. When a child is learning it is critical that the piano be kept at a pitch or the student will learn to hear the wrong things. These pianos need to be tuned at least two times a year. If the piano is not being used it still needs to be turned on a yearly basis.

Then there are the professional pianists, Most of these folks will have the piano tuned before every performance.

The answer to how often your piano should be tuned is very subjective. It depends on so many things. How cold or warm, how humid or dry its surroundings are and then there is the issue of the quality of and the shape the piano is in. These are just some of the things that affect a piano tuning.