Tuning is not the only maintenance that pianos need! As dust and other debris accumulates inside a piano, age and routine use wear down moving parts, and hammers compact and form deep grooves, various other maintenance tasks become essential.
There is no standard interval for these maintenance tasks; but as a general rule, the more use the piano gets and the more advanced the pianists using it, the more frequently they should be done. Many pianists are simply not aware of what a well-regulated and well-voiced piano feels and sounds like, and don’t realize what they’re missing!
As a general rule, any instrument that is being used by an advanced pianist, or for any performance purposes, should have voicing and regulation touched up on an annual basis. A thorough cleaning and regulation will generally be necessary every few years. If these tasks are done on a regular basis, the costs and time involved are much more manageable. In other cases where general maintenance has been neglected (even if the piano has been tuned regularly), it may require one or two full days of work to get the instrument back in shape. Such a job is more costly, but still more than worth it for the health of the piano and enjoyment of the pianist and audience.