Sometimes I see the same threads over and over for days and weeks. They keep popping up after I thought it was beaten enough. Some I find stay relevant like the one about memes and the other about people dying. Others I find that I stop looking at them after a point. It might not be a post count or page count but could be once the thread dissolves into something else.
For simplicity sake, maybe a general post count since I know many use 10 post/page and others have 20/page.
I find that after 100 posts I wander away from most of the threads, like 80% of them. What is your limit?