If your piano is new it will take a few months for its tuning to stabilise. This is because the strings are still stretching and the soundboard is still adjusting to the micro-climate in which it is sitting. New pianos are often bundled with 3 free tunings. The first one should be done immediately after delivery. We strongly recommend that the second tuning be done one month after the first tuning. The third tuning should then follow within 3 months.
After that, the tuning frequency depends on how much the piano drifts after being tuned. At the very least the piano should be tuned every six months even if it is never played. If the piano is played regularly, then more frequent tunings are recommended. Tuning drift is less in a piano that is tuned regularly.
Tuning is the minimum that should be done. It is like the basic servicing you do for your car. Beyond that, having a piano technician tune the piano regularly (even if it is only twice a year) means that the tech can help you monitor the condition of the piano and advise you about on-going maintenance and repairs.