We lock threads when we notice them somewhere after 100 posts. (You can see some get way larger than that until we get to it.)
It keeps the mechanics of the board moving more smoothly.....if too many get too large, they board moves slowly. I don't understand it beyond that. 100 is the magic number....probably by a decision.
So....when I'm on a forum, and I notice active threads (1st page or 2) over 100 posts, I lock them.
Hope that helps.
sg Love is PATIENT, love is KIND, LOVE never fails / DB since 2001