How Often Should It be Done
3 times a year will keep most pianos in good tune. All manufacturers recommend about the same thing. Some will say 3, some will say 4, some might say 2. It doesn't matter, the principals are the same. Results vary depending on how the humidity changes and how much and how hard the piano is played. It is extremely important the first year a piano is new to tune it at least 3 or 4 times, because of some issues of the piano "settling" and the strings stretching.
However, for a piano that is fully settled and is also equipped with a piano humidity control system, is is often quite satisfactory to tune them once a year.
Most people do not have the "ear" to recognise when a piano is out of tune. These people let their pianos go out of tune because they can't tell the difference. In the case of musicians, if they don't have the "ear", the usual reason is that they (or their parents) never kept their own piano in good tune, so they never learned to memorize the difference between in-tune and out-of-tune. Or sometimes they take their lessons from a teacher with no "ear" for tuning, so they never learn about it.
You might find yourself spending so much money trying to get a long-neglected piano back to where it will hold a tuning again that you may have wished you had kept it up in the first place, but it doesn't permanently damage anything. the majority of long-neglected pianos I see have other deterioration issues that have crept in (often concerning enviroment factors) that would have been "caught" at an early stage by a consciencious tech, and prevented from getting worse.