AOnce or twice a year.

For more information, here is an excerpt from and article on the PTG website:

©1993 Piano Technicians Guild

Your piano is an investment in your future. It can bring you and your family a lifetime of music, adding immeasurable joy and beauty to your home. Since it is also such a large investment, it should be maintained with the utmost care. Regular servicing by a qualified technician will preserve your instrument and help you avoid costly repairs in the future. See complete aricle at www.ptg.org