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:
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:
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.
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.
Pianos go out of tune primarily because of changes in humidity. Tuning can be made more stable by installing special equipment to regulate humidity, inside or underneath the piano. There is no evidence that being out-of-tune permanently harms the piano itself. However, a long-term low-humidity environment may eventually crack the soundboard and warp keys and other wooden parts. In particular, pianos located in arid climates and otherwise very dry rooms (for example, a cold climate with an extended heating season) require special attention to humidity control.
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Notes From the Piano Bench
Questions and Answers and updated factoids concerning pianos in the 21st Century.