Generally a piano should be tuned once a year.
However piano's vary as does the frequency of a piano's usage.
eg. If a student practices, or a piano is played for more than, or approx 2hrs per day the piano should be tuned at least twice a year.
Piano Teachers may require additional tunings per year due to the sheer number of students with different skill levels playing the piano on a daily basis.
New Piano's Additional tunings are required in the first year, this is particularly important as the piano strings have not fully stretched or settled down.