I don't know, they shouldn't get moved to a group with less userrights just because they didn't use the forums.
BTW, the forums do log in some database fields called "last_visit" and "last_activity" as a UNIX timestamp. The difference between the two I didn't bother to check out; but on most of the accounts I saw, "last_activity" was a later timestamp than "last_visit".
We can use those to gauge a person's activity level if we need (I can't remember if a display for it is part of the forum software, but that can be added).
Edit: last_activity is the last click on the forum (gets recorded for online list); last_visit is set on user login and other things like postings, etc.
the reason you can have them far apart (days) is because of the login cookie.