I would think that in approaching this you need to look at why this forum was created. Which is to help out users control their big boards.
Based on that I would say based on my experience, I didn't start noticing some issues until I received 1 1/4 million posts and about 200 users online at a time. Things started to slow down.
So I think any condition in which you have a large number of posts or users and your forum is not performing as you think it should, and need some assistance in improving the speed or reliablity of you forum you should be able to post here.
Now obviously someone with 75K posts and 200 users probably needs not apply.