Several factors affect piano tuning.
With our hot, humid summers and cold, dry winters I recommend tuning twice per year. Ideally the timing will coincide with the start and end of the heating season, but really anytime is best. Once per year is the minimum.
Pianos go out of tune whether they are played or not. The longer the tuning is neglected the further flat the pitch will go. It is a complex problem, but the more times the pitch of the piano is way off and pitch raises are required, the more long term damage is done to the tightness of the tuning pins and other components of the piano. Pitch raises also carry the risk of string breakage during the tuning process, resulting in costly repairs.
A pitch raise is essentially an extensive tuning that must be done on a piano that is very out of tune, such as one left un-tuned for several years or more. When a piano is left un-tuned for a long time, so many strings become out of tune that the tension across the entire soundboard changes. In this condition, tuning one string will affect the tuning on other strings. A vicious cycle ensues where previously tuned notes lose tune every time a new note is tuned, like a dog chasing its tail. To correct for this, the tuner will do a pitch raise, where every string's tension is adjusted in one pass, to bring the tension on the entire soundboard close to what is desired. Then, often after a resting period, the tuner can perform the precise tuning of individual strings without affecting the others. Several passes may be needed before the pitches stabilize. Click here to learn more about pitch raises from the Piano Technician's Guild.
There are several things to try, from nearly zero cost up to hundreds of dollars.
To determine a piano's ago you need to know the manufacturer and serial number. Manufacturer is usually found on the fallboard, that is, the lid that flips down to cover the keys. The serial number is found by lifting the lid on an upright piano, and by looking near the tuning pins on a grand. There are many online resources available to try an internet search using [manufacturer name] [serial number] and [your serial number].
A glass of water would be nice, thank you. ;)