I always recommend having your piano tuned on a 6 monthly basis. This will ensure each tuning lasts as long as possible. Pianos are under an immense amount of pressure and will pull themselves out of tune even if not played. The smaller the pressure change during tuning, the longer the tuning will last. Twice yearly tuning also accounts for the changing seasons we experience in the UK, with very humid summers followed by the central heating period, which is very dry. (See piano care – humidity)
Another important reason to recommend regular tuning is to pick up on any adjustments that may need to be made along the way – these include regulation tweaks and touch adjustment. Without these being picked up early, the action can start to wear it’s felts down, eventually requiring repair or even restoration which can be extremely costly.
If your instrument is an older piano that has not been tuned for some time, it is often the case that I will recommend tuning it at a semi-tone flat from pitch. This is because raising a piano that’s a long way flat up to pitch will cause either the piano to go straight back out of tune, and/or the possibility that some strings will break along the way. If I assess that the piano is suitable for bringing back up to pitch, this can be done in 2 visits, or gradually over each 6-monthly tuning.