This is a tricky question - the standard answer is once or twice a year depending on how picky you are.  Once a year is always adequate to compensate for seasonal changes in humidity and if it's tuned at approximately the same time every year it will remain at a consistent pitch.  Tunings do fluctuate with varying humidity as the soundboard shrinks and swells, which in itself may not be noticeable unless the unisons (more than one string on the same note) drift also.  So more than once a year can keep the unisons better in tune throughout the seasonal humidity changes.  Letting the piano go for more than a couple years may result in loss of pitch - the whole piano may go flat - which may necessitate an extra pitch adjustment - which is a rough tuning to get the pitch close enough to do a finer tuning.  The problem with having to do a rough pitch adjustment is it compromises tuning stability because the more you move the tuning pins and stretch the strings, the less predictable the long term stability will be and the piano will almost certainly go out of tune sooner than if it were regularly tuned.

That being said, I'll sometimes recommend tuning at less frequent intervals than once a year.  If the piano is older than 50 years or so, or a very small piano, often those instruments are not as receptive to fine tuning and will always sound a little off - even the unisons - due to older strings or just an overly compromised scale design.  Older and smaller pianos are also generally less responsive to humidity changes and don't tend to change tuning as much with humidity fluctuations, so even though they don't tune up as well to begin with, they can be more stable.