When a piano is brand new, (or has brand new strings), the new strings will stretch more, and initially go out of tune faster. This is why new pianos, (or pianos with new strings), often need to be tuned up to 4 times in the first year for best results.

If your piano has not been tuned in many years, and has a major adjustment to pitch, it may need to be tuned 2-3 times over a year to get the best results as the strings will stretch and the piano will settle.

Changes in climate, specifically changes in humidity and temperature, also affect the piano.
For most pianos, it is recommended that your piano is tuned at least twice a year. However, many people choose to have their piano tuned only once a year.

A piano may need additional tuning if a piano is moved, or if the piano is heavily used, (such as piano teachers, or music studios).