How far does it break it down?

How far down does the multi-db plugin break down the database? For example, I have one site which I'd like to use it on that will have only about 50 different blogs under it; however all of the blogs will have 15K+ posts, and some of those blogs will have hundreds of thousands of posts, all with at least 5-10 custom fields each. Will the plugin break down those tables to a different DB once they reach a certain point?
Also, when does it automatically determine when to start adding information to the next database, and is that adjustable?

Looking for some clarification on how exactly it works, please! (?)