Simple answer: Every six months if you have a “good ear” and need it to be “right,” once a year if you just want to maintain it at a reasonable level, acceptable to the average listener.
Better answer: Depends on many factors. It is not unusual in a professional situation, (such as recording or performance) to tune the piano once or even twice per day. Many hotels and restaurants tune their pianos once a month. Manufacturers recommend that new pianos be tuned up to 4 times in its first year then twice a year, minimum of once a year. Older pianos that are stable and have had regular tuning can sometimes do fine with tuning every 2 or 3 years, this however all depends on the climate conditions the piano lives in. We will assess your situation and give an honest recommendation, based on your piano and your needs.