How long it takes to "break in" a Brooks is more dependent on how long it takes to adapt to the saddle rather than the saddle adapting to you. My first Brooks was a B-17N on my touring bike and it was instantly comfortable out of the box. I can say the same for the Brooks Pro, another B-17N, another Brooks Pro (which my daughter currently rides), a Brooks Pro Alpe D'Huez (which my wife is currently trying) and the Brooks Swift (with Ti rails) that I bought in October. Basically, I've never experienced the "break in" period on any Brooks I've used.

But I've always ridden a much firmer saddle than most people do. If I'm going to have trouble with a saddle, I've noticed that the softer ones cause me more issues than the harder ones.

I will say that I also have plastic saddles on mountain bikes. I've found that Brooks don't work that well on a mountain bike...they are too wide at the back to allow for sliding off the back of the saddle. Since I have plastic saddles on my mountain bikes, I can do comparisons of Brooks to plastic and I can tell the difference. For me, the plastic ones don't give under the rider and thus feel wider between my legs. There is a subtle resistance to bending that just makes the plastic ones not as comfortable. It's not a huge difference and riding one or the other wouldn't be a problem...I can adapt to anything... but it is noticeable.