Answer: If you ask the average tefillin-wearing Jew on the street, he will tell you that you have to have them checked twice in seven years. (How you ever heard that?) This is what I always heard too, and therefore, I was surprised when one of the most reputable ‘sofrim’ (scribes and experts) in a large religious community told me the following matter-of-factly, when I brought my tefillin to him for my semi-Sabbatical check-up: “You know, you don’t have to have them checked!” (Ed: The first time I went to him, I took his word for it, did not ask any question, said thank you and left!)
Having since seen the Shulchan Aruch and Mishnah Berura, I now see what he meant: The Shulchan Aruch rules that tefillin that have a status of ‘kashrus’ (i.e. they were once certified kosher, and since then, nothing has changed to upset that status), do not need to be checked – ever! What constitutes a change which could upset that status? For one, not wearing them regularly! That is when the halachic standard of having them checked twice in seven years applies, because since they are not under constant use, they may have suffered from discoloration, rotting, etc. (Note: There are other factors which can upset the status of ‘kashrus’ immediately - e.g. if the stitches became undone, if they fell into water, were exposed to intense heat or moisture, etc.)
So, why is it common practice for people to check their tefillin according to schedule, even though they are worn daily? I believe that this is because the Mishnah Berura writes that it is proper to have them checked in all cases, because, over time, they may become affected by perspiration. That, I believe, is the source for the common practice of regularly-scheduled tefillin checking.
So, back to the sofer, what I think he was saying to me was this: Listen, if you wear your tefillin regularly, then you are not required by halacha to check them. He probably feels that it is his duty, as an expert, to teach people what the halacha says, because they may not know this. If someone says to him, “I know that I am not required to, but I wish to do what is “proper”, then he will check them.
[Ed: Subsequently, I saw in another respected contemporary halachic work (Piskei Teshuvos), words which I would like to paraphrase for you. You may take them to heart and discuss them with your rav, if you have any further questions: “Regarding the tefillin that we have today (I think he is referring to the ‘gasos’, by the way – ed.), which are made of high quality materials and workmanship, and do not seem to show any outward signs of wear or damage, one who relies on the straight ruling of the Shulchan Aruch has not lost anything (i.e. has not gone against halacha), as long as they have not undergone anything specific which would upset their status…” – Incidentally, he also writes about mezuzos in a similar vein, but that is not our topic here…]