Like the title says: how often should one have his tefillin inspected, and why was such a frequency chosen?
Here is an article that discusses checking tefillin. See the article for more details and sources:
The Chayai Adam (14:20) says that even Tefillin worn daily should be checked twice in seven years.
According to strict Halacha, Tefilin does not have to be inspected ever. Proof of this is a Yerushalmi Eruvin 59 Halacha 2 from Hillel Hazakain.
In the Teshuvos HaGeonim it says that so long it is being used constantly there is no need to check, however if it is only used from time to time it should be checked twice every 7 years like Mezuzus.
HaRav Ovadia Yosef Shlita says that it should be checked twice every seven years.