I'm not speaking as a pastor because I am not one. In fact, I've never even been called to preach.
This is simply my personal view and doesn't represent the view(s) of any church of which I may have been a member (either in the past or in the present).
First of all, as you indicated in your OP, I believe that there are some overriding factors in an individual member's life that ought to be considered regardless of the length of time on which a church decides is "too long" for a non-attending member to be considered as "active."
These would include:
1) Sickness (and here I'm taking about a situation that would result in someone being a "shut in" as opposed to a person continually saying "Well, I got a fever every Sunday morning for the last six months that miraculously goes away every Monday morning!");
2) A permanent type of physical disability (e.g., polio) that continually makes it almost impossible for that individual--regardless of age--to attend on any regular basis;
3) Military service that results in a person being stationed too far away from the church and thus unable to attend on any regular basis. (NOTE: However, I do believe that in such a case as this that a person ought to seek out a church of "like faith and order" [Whatever that actually means!] that is close by that person's military duty station and join that church while he/she is stationed there.);
4) Being gone for a lengthy period of time due to work or educational requirements. (EX: A member's employer sends him to another location too far away for some legitimate business-related purpose, or a college student being involved in a "study abroad" situation.); and
5) Having a type of job that may entail being "called in to work on an emergency basis" on several Sundays. This would be something such as being either an officer of the law, a fire fighter, or having some sort of medical profession (e.g., doctor, nurse, EMT, etc.).
But here are some other issues I believe ought to be factored in on this matter:
1) Should the number of "missed services" only include the regularly scheduled Sunday morning services? (EX: A member has a job that requires him to work on Sunday mornings, but is able to [and does so faithfully] attend either a Sunday evening or a mid-week evening service during that week.)
2) What are the consequences of being an "inactive" member? Does he lose his right to vote as a member on any/all decisions that are brought before the church body during that period of time where he's considered as "inactive"? One church of which I was a member years ago that held to a "closed" communion concept (i.e., Only the members of that particular local church were invited to partake of the elements of the Lord's Supper.) had in its by-laws that "inactive" members could not participate in its communion service as long as they were considered as "inactive."
3) Who decides on whether or not a member is "inactive"? Is it upon a majority vote by the members in a public business meeting? Is it a ruling by the pastor & deacons only? Etc., etc.
4) What procedures must an "inactive" member follow to then in order to subsequently be considered as "active"?
Finally, as in matters of church discipline, it ought always to be the desire of a church's membership to lovingly seek out the restoration of any and all members who appear to be apathetic towards their church rather than merely to wish that member to be considered as "inactive."
Before the issue of whether or not a member ought to be considered as "inactive" even comes up for discussion, someone (maybe the pastor himself, maybe one or more of the deacons, or maybe just a "regular, non-office-holding" member) ought to have sought out this member and see what's going on in that member's life.
It's entirely possible that something(s) have come up in that person's life that nobody else in the church was aware of.
Moreover, it may be that the church needs to help out this member in some sort of tangible way but wasn't aware of that member's need(s).
Forgive me for being so lengthy and somewhat detail-minded in this post, but I do believe this is a matter that many churches either overlook entirely or are very haphazard (and maybe even unbiblical) in its implementation.