The leather will eventually mold to the shape of your butt. That is when it is considered broken in. How long that takes depends on the person and the individual saddle. Some Brooks take longer than others to break in.
I have four B-17's on my everyday bikes and I love 'em. However my "go fast" bikes sport lighter saddles which are not as comfortable but (pun) I tolarate for the weight savings.*
*Not that I think I can actually go fast, or that saving 300g will make me a better rider.
