The scrolls inside the Tefillin will decay with age if not used on a regular basis. Therefore, tefillin which have been stored without use for an extended period must be inspected by a competent scribe twice every seven years. Tefillin which are used regularly (good for you!) technically do not have to be checked at all. This is provided that the exterior box has not been damaged, torn or soaked.
Nevertheless, it is proper to check tefillin occasionally, even if they are in constant use. “Better safe than sorry” — can certainly apply to such a special Mitzvah which has such a profound affect on a person’s daily life. Many Chassidim have the custom of checking their tefillin (and Mezuzot) every year during the month of Elul — adding another mitzvah to their arsenal in preparation for the Days of Judgment.
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Article source: http://www.askmoses.com/article.html?h=149&o=286