Almost all manufacturers recommend having your piano tuned once every six months. It's often possible to stretch that to a year without adverse results; however, going past that point can mean a tuner will have to do pitch raises to get the instrument back up to pitch - so you really don't save money by postponing tuning plus you increase the risk of breaking strings during the pitch raising process. Why six months? Piano soundboards are hygroscopic, meaning they absorb or release moisture when the relative humidity goes up or down. This causes slight changes in string tension, resulting in changes in pitch. Tuning every six months corrects this.
Pianos placed in environments where the temperature and humidity fluctuates often will require tuning more often. Churches that turn the heat or air conditioning up or down during the week are a prime example. Pianos placed in close proximity to HVAC vents can experience the same issues. Steady temperature control, away from drafts and direct sunlight will help your piano stay in tune longer.
Sometimes placement of a piano cannot be reasonably changed. In that case, we recommend purchase and installation of a Dampp Chaser system which helps mitigate swings in relative humidity around the soundboard of a piano. Email or call me for additional info.
Placing pianos on an outside wall - I'll discuss that topic in a future post.