This article is from the Piano Purchase and Maintenance FAQ, by Isako Hoshino email@example.com with numerous contributions by others.
For a brand new piano, the general recommendation is to tune
it about four times during its first year and twice a year
thereafter. This is also a good rule-of-thumb for older
pianos which were moved into a different climate condition.
Most other pianos generally need to be tuned about twice a
year. However, some pianos may require more frequent tuning,
and some less. The frequency will vary depending on the age,
model and the environment of the piano.