Frequency of tunings depends on several variables. The humidity changes are the biggest cause of the piano going out of tune. This won't eliminated the need for tuning but it will help.
Some pianos go up and down in pitch together and stay relatively in tune.
The bass stays in tune and the middle goes up or down and creates bad octaves. Playing the piano, especially heavy playing can tend to knock the unisons out. The unisons are the tuning of the three strings for each note. I play heavily when I tune, but constant heavy playing will cause the piano to go out of tune sooner. Some people are sensitive musicians and hear the piano going out of tune much sooner than others.