Here's the million dollar question.
What's the guess on an upgrade time from 2.1 to 3.1 1 with 1.7M posts and 60K users and 5000 uploaded files (Vertex dedicated server)?
Couch upgraded mine in less than 12 hours. Mine was 2.2 to 3.1, with 1.5M posts, 45k users and 17gb of images.
The only complaint I have is that it took nearly a month to get this upgrade performed (I paid for the professional service). The actual upgrade once it took place was very quick. There were some issues with my upgrade though, which Couch fixed very quickly.