How often to check Mivtzaim Tefillin

Question:

I own a pair tefillin which I use only for Mivtzaim, which I usually do once a week on Fridays. I would like to know if I must check them every year, or how often they should be checked?

Answer:

It is not a requirement check them annually, although certainly one is encouraged to do so, just as you are encouraged to check your own tefillin once a year. At the very least, they must be checked every 3 ½ years. [The tefillin that you wear daily from letter of the law does not ever require checking [so long as nothing potentially damaging was witnessed to happen to them], however, the Mivtzaim Tefillin which are only worn once a week must from the letter of the law be checked once every 3 ½ years.]

Sources: See regarding checking Tefillin twice in seven years if worn only on occasion:  Admur 39:11; Michaber 39:10; Orchos Chaim Tefillin 29; Haittur Tefillin Chelek Hateshi; Shaareiy Teshuvah 153; See Regarding that we don’t know the definition of “occasion” and hence any Tefillin not worn daily is to be checked every 3.5 years: Mateh Yehuda 39:8; Kol Yaakov 39:39