The answer to this depends upon several factors.
1. Every time a piano is disturbed or moved it will probably require re-tuning. Grand pianos are more susceptible to this than uprights.
2. How often is the piano played? A piano that is in constant use will require re-tuning more often than one that is infrequently used. Also, a piano that is played with vigor will require tuning more than one played softly.
3. How important is the sound quality? When sound quality is very important, a piano will require tuning before each performance.
4. In what environment is the piano kept. Stability of temperature, and humidity, are very important, as fluctuations of either will effect the tuning of the piano. When we tune our pianos, for a performance, we prefer to deliver the piano as early as possible, to allow the piano to stabilize in temperature, and we try and tune it as near to the performance time as possible requesting all spotlighting is turned on.
5. What condition is the piano in, and what quality of manufacture is it. A well-maintained piano is likely to keep its tune longer than one in poor condition, just as a high-quality piano will be better than a lesser quality piano.
To summarize, in general, a frequently played piano, in a central heated domestic home, will require tuning about two times a year. A less frequently played piano may be tuned once a year. A well-played piano, used for stage performances will require tuning prior to every performance, possibly even twice a day.