Your piano will stay at the correct pitch and sound best if it is kept in tune regularly.
How often should my piano be tuned?
That depends on the piano and how much it is played. All pianos drift out of tune with playing and particularly with temperature and humidity changes. How much the piano changes depends a lot on the actual piano, some barely move and others will be noticeably out of tune quite quickly. If you are a regular player it is preferable to tune the piano every 6 months. keeping a piano in tune minimises the amount the instrument drifts and makes for a more stable and pleasant tuning overall. If you play only occasionally it is good to get the piano tuned once per year. If the piano is left for many years between tunings the overall pitch will likely drop quite a lot; the piano becomes harder to tune, will require a pitch raise tuning and will not stay in tune for as long.
How much does it cost?
The usual fee is $165. Pianos which have not been tuned for a long time may get very flat; in which case the piano may need to have a first tune called a "pitch raise" before it can be tuned properly. A pitch raise allows the piano to settle enough at the higher pitch to allow a fine tune to settle in at the new pitch.