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:
Once Tefillin are known to be Kosher, they Halachically never need to be checked, assuming they are worn regularly. If they are worn infrequently they must be checked 2 times in 7 years. Shulchan Aruch (Orech Chaim 39:10)
However, the Aruch Hashulchan says that ink does peel off the parchment, and therefore Tefillin must be checked periodically. There are different opinions how much is periodically (see the article).
A Mezuza is required to be checked twice in 7 years, and it is possible that is where the number for checking Tefillin came from
If visible damage was done to the Tefillin they must be checked.
The Kitzur Shulchan Aruch (128:3) says that it is considered meritorious to check your Tefillin every Elul.
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.