Most manufacturers recommend tuning your piano two to four times a year. It is also important to note that when buying a new piano manufacturers require that your piano is serviced at least annually by a qualified technician to keep the warranty in effect.
Your piano, just like every piano, is designed to sound its best when tuned to A-440 (the A above middle C vibrates at 440 cycles per second), the international standard pitch. It has been designed to perform at a specific tension, and when stretched beyond or dropped below this tension, pitch adjustments are required to bring it back to A-440. It's important to remember that maintaining your piano at standard pitch allows you to play along with other instruments which are all designed to this same standard. Through neglect, pianos may deviate from this standard, making them unsuitable to play with other instruments and causing them to lose market value. In addition, lower pitched instruments can compromise the pianist's ear training It's important to note that pianos do not go flat or sharp uniformly. Some strings will invariably change more than others.
Changes take place because your piano's overall pitch is dependent upon changes in the relative humidity. In some temperate regions of the country, the relative humidity increases in the summer resulting in a higher moisture content in the soundboard and a higher string tension (pitch). In the winter, when heating systems dry the air, the soundboard loses moisture and contracts, causing the pitch to drop. The drop in the winter tends to exceed the rise in the summer, so the net result is a drop in pitch each year that the piano isn't serviced. You can greatly increase the stability of your piano's pitch by maintaining a relatively consistent humidity level in the room.
When the tension of each string on a piano is raised back up to pitch, the additional load on the piano's structure causes the pitch of previously adjusted strings to change. The only way to achieve a fine, accurate tuning on a piano is to have the tension of all the strings as close to their proper place so that altering the tension of one string would not affect the others. Therefore, a piano must already be fairly close to standard pitch in order to be finely tuned.
If a piano has drifted two cycles per second or more, a separate pitch raise is advisable. Like your car, your piano is a major investment that deserves to be protected by regular servicing.