My Brooks B-17 broke in over the course of about 500 miles. But by 200 miles it did start to feel better. That said, my commute is 22 miles each way. So to me the break in seemed to go very fast. Indeed, out of the box I thought it was hard but not intolerable. I now have about 2000 miles on it and it actually got too soft so I tightened it a bit.
Also, I followed Brooke's advice and added proof-hide to the saddle once a week for the first three weeks. Not sure if that accelerated break in.
I also had a B-17 narrow on a road bike. After 200 miles I replaced it with a synthetic saddle. To me the B-17 narrow on a road bike pushed into my taint too much and was unbearable.