It's usually less about quantity than it is quality.
If I glance at a topic and the last 50 posts or so are raw stupidity vomited forth onto the screen by people who should probably have their Internet rights revoked, I generally close the window and never look at it again. Doubly so for topics that have pretty clearly devolved into 2-3 people just shit-slinging back and forth at each other.
But if a topic has managed to stay relatively intelligent and there's still a point or two I'd like to make that no one else has brought up yet, I'll usually chime in even if there's been a ton of posts beforehand.
"Wall of Text'D!"
--- oldskoolplayr76
"POwned again."
--- blight family