A regular tuning schedule is essential if a piano is going to be enjoyed. An out of tune piano is no fun to play, and letting a piano go for a number of years can make bringing it back to its potential much more difficult.
In general, most pianos should be tuned either yearly or every six months. A piano that is not being used currently but may be used at some date in the future should be given a minimum of a tuning every two or three years.
For a piano that has been neglected for a long period of time, an initial "pitch-raise" may be needed to bring the strings up to tension before the fine tuning is performed. Once a piano has been brought back up to standard pitch, it's best to tune the piano again in six months, then again after a year has passed.