Generally, as soon as you tune your piano, an hour later it will fall out. Thats why concert pianists tune their pianos before practice,, and again before their performance (same day).
since none of us really need that level of precision during our daily lives, we get ours tuned every so often.
New pianos tend to be tuned more frequently (every 3-4 months). after the first year or so,, your piano will start to hold its tune, and you can pass your tuning about every 6 months (at least twice a year)... however, like the previous post,, humidity and amount/level of play will factor these numbers.
Keep this in mind,, the more often you tune, the greater your pianos tuning stability will become.
you're the best person to judge when your piano needs tuning,, no one else. expect it around every 2-6 months.