A common question is "How often should a piano be tuned?" Although the textbook answer is "every six months", the better answer may be "it depends". Key factors are temperature, humidity, location, piano age, type, usage, and what the person playing likes to hear and is able to discern regarding the tuning.
Key factors include temperature and humidity which may change often in some homes or auditoriums. Seasonal changes tend to be a contributing factor also. The piano owner along with the tuner/technician can determine the optimal tuning schedule.