Some people say their pianos require tuning once in 6 months, while others say they need to fix theirs about once a year. In reality, there is no fixed time duration to tune a piano. It depends on the trained ears to determine when a piano needs to be tuned.
For example, in case of professional performances and recordings it is not uncommon for a piano to be tuned almost twice in a busy week. Piano manufacturers recommend tuning a piano every 3 to 4 months. The close frequency of tunings on a new piano helps stabilize the strings. Once the strings stabilize pianos can do with tuning every year.
Humidity fluctuations is the number one cause of changes in piano tuning. When the soundboard, pinblock and bridge are in a moist environment, the wood cells absorb the moisture and swell up, and as they expand they pull the strings tighter, causing the piano to go sharp. Moving always knocks a piano out of tune.
Tuning pianos that are prized by their owners are tuned regularly, usually once every six months for domestic pianos, and always just before a performance in concert halls.