A. Pianos are both heavy and sturdy on the one hand and yet rather sensitive and delicate on another. The nicer the instrument, the more care they may require to show off their greater capabilities. When new, pianos tend to drop flat fairly quickly as the new strings actually stretch some and the new soundboard is compressing some under the incredible pressure that it must bear up under. (Some manufacturers will recommend that their instruments be tuned as often as 4-6 times during their first year of service.)
Aside from those 'breaking in' processes, the day to day and season to season changes in climate (pianos are more humidity sensitive that heat sensitive regarding tuning) cause the soundboard to swell and shrink, increasing or decreasing the crown (or belly). This change of the soundboard crown will increase or decrease tension on the strings which causes the piano to creep sharp in humid weather and drop flat in drier weather. The piano is subject to the macro changes of season to season, the lesser changes of day to day weather (showers today, sunny tomorrow) and to a slight degree, even the micro climate of your home (cooking, laundry, empty home when everyone is at work or school.) This complex combination of constantly fluctuating conditions keeps the more sensitive soundboards changing all the time. On a day to day basis under normal conditions, most people will not notice the change in the tuning of their piano. But over time, the scale becomes more and more distorted until one day you notice that your piano is out of tune. It really happened very normally, little by little, over a period of time. Some people will notice the distortion before others and some pianos (especially the lower quality instruments) are not as sensitive to those changes in environment.
Tuning, then, is the re-ordering of the scale of the piano and adjusting the pitch up or down as necessary. How often you have your piano tuned is mostly determined by what demand you place on your instrument. But the minimum service required has to do with keeping the piano more or less within the tension range that it was designed to operate under. A piano should be tuned as often as required to keep the pitch from falling no more than 8-12 cents flat or rising no more than 6-10 cents sharp. (A 'cent' or '¢' is 1/100th of a half step.) In our part of the world (on the Cumberland Plateau in eastern middle Tennessee) that requires tuning most home pianos every six months or so (minimum of once a year). That becomes our standard expectation as to how long a piano should stay 'in tune.' Any drastic changes in weather conditions will have an effect on the piano's tuning. Due to structural stresses on a piano, it will also need to be retuned shortly after it is moved. (We recommend 3-4 weeks.) A piano that is neglected (not tuned regularly) will fall significantly flat over time and require extra care (and expense) to get back up to pitch, and in some cases will fail such that it cannot be tuned up to standard pitch or will not hold tune at all.