Many factors contribute to how quickly a piano goes out of tune. Some of these factors include humidity and temperature changes, slipping of tuning pins, stretching of the strings, and the amount as well as intensity of use. Seasonal changes compromise your piano's tuning stability. Excessive dryness or moisture in the air will cause your piano to slip out of tune even if it was tuned recently.
This is why piano manufacturers suggest having your piano tuned every six months for optimal performance. That same excessive dryness and moisture can also cause internal damage to your piano. Many manufacturers recommend installing a climate control system to protect your investment.
To many pianists, a piano's touch is as important as its tone. When a piano begins to lose its mechanical efficiency, it is said to have gone out of regulation. Adjustments which were set at the factory no longer are accurate because of the compression of felt parts which serves as cushions to all the contact points in the action. Exacting measurements and corresponding adjustments are needed at this time to put the piano back into proper regulation.
Your piano action has thousands of individual parts, and after decades of use, breakage may occur or parts may simply wear out. The good news is that most of the parts found in a piano action (the working mechanism) do not often break (even after a century or more of constant use!), and those parts that do happen to break or wear out may often be replaced with parts available to the professional piano technician, or repaired to like-new condition.