You should have your piano tuned at least every six months. New pianos should be tuned three times within the first year. After this is completed, a new piano should hold its tune properly and may then go onto a two-times-per-year tuning. schedule.
Tuning your piano on a regular basis is no different than taking your car to the service department on a pre-planned schedule. The bottom line is...the instrument will perform better and be worth more money when it comes time to sell or trade it in.
Although the older, full upright pianos are designed to be tuned to A-435, most pianos are designed to be tuned to the standard A-440 pitch. Stretching the strings to meet this pitch is relatively easy. Keeping the strings at this pitch is easier said than done. If a piano is left for long time without being tuned, extensive work may need to be done to correct the problem. If the pitch drops significantly, say even a quarter-step, the piano tuner will usually recommend a pitch raising. This involves stretching all of strings back to the A-440 pitch and then re-tuning the instrument until it stabilizes. This may take several hours to complete resulting in a higher fee than a regular tuning.
A Better Way...Tune your piano at least every six months so you can enjoy the instrument and keep the maintenance bill down to a minimum. Keeping your piano in tune can only be accomplished if the customer stays on the twice-per-year schedule and when an experienced piano technician works on the same piano consistantly. Good maintenance is a combination of the piano owner's scheduling regularity AND the piano tuner's level of expertise. Both of these factors equal a well-tuned piano.