This will depend on circumstances but, for most pianos the ideal is twice a year although once a year is a minimum. Tuning your piano regularly is essential for maintaining the pitch and string tension within the piano.
The pitch of the piano is also affected by changes in weather, both temperature and humidity. If the piano is moved, it is likely to need tuning. Regular tuning is required if you accompany other instruments or singers. The piano should always be tuned before a concert.