I have an old one (40-50ish years old) on one of my bikes, but I had to do a bit of work to get it comfortable (wetting and re-shaping, then leaving out in the sun to dry, followed by an application of Proofide, and then lacing the sides together), and I've been playing with the idea of getting a new one...however, I'm concerned about how long they stay firm. I'm used to some pretty firm saddles, like the Gilles Berthoud Aspin, and it bothers me when the sides of the saddle flare out. They rub on my thighs, and all that. If a new B-72 is likely to start sagging and flaring fairly quickly, I might just stay with this old one as long as I can.

With every Brooks I have ever tried they take a while just to break in when new. I haven't had a B-72 but a B-17 takes twice as long as any Selle Anatomica I have ever had. The leather tends to be thicker on the Brooks as well. Lacing an old brooks is pretty common place but normally seems to not be necessary till it has pretty much used up the adjusting bolt. Still the Imperial ships with a set of laces in the box.

So I would say you should have several thousands of miles before it will sag enough to be concerned with.

4,000 miles on mine, still going strong. Took about 150-200 miles to properly break it in.