Six months to a year. The only reason pianos go out of tune is not because it is played alot, it is due to the humidity changes effecting the wood. The main changes in humidity occur twice a year in winter and summer. So it is best to have your piano tuned twice a year. You will notice the piano going slightly out of tune after around 6 months. If you can wait a year, then it is advised to have it tuned before the end of the year. However, you have a piano which is slowly going out of tune after 6 months.