Here is what just some of the major piano manufacturers recommend about tuning your piano . . . .
Yamaha (also Bösendorfer pianos)
"Tuning means correcting the pitch of every note by retightening the strings. Each piano string is normally stretched to a pressure of about 90 kilograms (198pounds). Eventually, though, it will stretch further with use and lose some of its tension, causing the piano to lose its correct pitch. The strings need to be tuned a minimum of twice a year to restore them to their proper tension."
Steinway & Sons (also Boston Co. pianos and Essex pianos)
"With normal use and conditions, a piano should need only periodic tuning, perhaps twice a year. For reference, a concert piano is tuned before every performance and a piano in a professional recording studio is tuned several times per week. Tuning is a craft practiced by professionals and, for best results, no one other than a trained technician should tune your piano."
"All pianos need to be tuned regularly. The amount of time between tunings for a fine instrument depends on many factors, especially the stability of the temperature and humidity and the amount of use the piano receives. In general, Kawai recommends 2 to 4 tunings per year. However, your piano technician can best recommend the appropriate interval for your specific environmental conditions and use."
"All climate changes - shifts in humidity and temperature - affect your sensitive instrument. The mechanical stresses on your instrument as well as temperature and humidity changes have an impact on the tension of the strings. Therefore, over time the tuning starts to become irregular. Your piano should be tuned at least twice a year. Treat your upright and your grand piano to a regular tuning service . . . This preserves the value of your instrument and your musical enjoyment. It's worth it."