Not all pianos are the same, and not all pianos are used the same - here's a chart I use to give piano owners and players an idea of what would work best for their situation:

NEW PIANOS:
A new piano should be tuned 4 times the first year, and probably twice a year after that - but much will depend upon how the piano is used - see below:

HOME PIANOS
Advanced Use - used hard 4 to 5 hours daily - Service Piano every 1-3 months.
Medium Use - used 2 to 3 hours daily - Service Piano every 3-6 months.
Light Use - used 1 to hours daily - Service Piano every 6-8 months.
Occasional Use:  Randomly played - Service Piano every 6-12 months.
Never -  If a piano is not really used, then have the piano tuned once a year to avoid structural damage caused by the lowering of the pressure caused by the piano strings.

INSTITUTION PIANOS
Concert Piano - service before each concert. (This is a special service that usually requires 3 to 4 hours to prepare, and a touch-up check at intermission)
Professional Practice - used hard 4-8 hours daily. Service piano every 7 to 30 days.
Teaching Piano - used 3-6 hours daily - Service piano every 1-3 months.
Church Piano - Service piano every 3-6 months.
School Piano - Service piano 3-4 times a year. Schedule around performances and when air conditioning/heating is turned on or off.