If you have any doubts or concerns, or if you have a reason to suspect something is amiss, they you should have your Mezuzos and Tefillin checked right away.
By checking your Meuzuzos regularly you will prevent serious problems from occurring as well as a potential big expense of having to replace your Mezuzos.
If the letters start to grey or if the letters and crowns begin to form a small crack and break this will cause the Mezuzah to become not Kosher, if caught in time it can be fixed, however if it goes years without attention chances are it will be too late the breaks will be to big to be able to fixed and it will have to be replaced.
As well unfortunately there are Mezuzos and Tefillin that were Not Kosher from the get go, and when a Sofer Checks Mezuzos or Tefillin there are many potential problems he is looking out for, such as: Letters, Words or/and Tagim that are Missing, Extra, Substituted, Touching, Broken. Improperly Spaced, Deformed. Faded, Cracked and weather damaged. As you can imagine that is a a lot to focus on while checking a Mezuzah or Tefillin.
Very often when a Mezuzah has a lot of one particular problem such as problematic spacing the Sofer will be giving most of his attention to fixing the problematic spacing and not be giving 100% to other areas such as missing or added letters... Therefore the more often a Meuzah or Tefillin is checked the greater the chance all problems if any will be discovered.