It would be nice if everybody was a millionaire. Frequency of tuning would depend on your economic priority and musical needs. If you are a professional musician, you would tune more frequently than ordinary people no matter what. If you have to worry about what to eat tomorrow, you obviously can't spend any money no matter how badly the piano is in shape. For a middle class ordinary household, one tuning a year would be reasonable.
Professional pianists tune their piano everytime before the concert.
I personally tune my piano every time I hear something wrong. In English Piano strings, which are made from steel, are under a tremendous amount of tension. Change in temperature and humidity will only contribute to further stretching. Each individual strings will stretch at random causing single note to produce two or three different tones. You are not supposed to hear a chord from a single note.