I've had guns that took a few hundred rounds to break in up to 10,000 or so. The longest one I ever had to break in was a new Bl4 Beretta 12 gauge that was so tight that it took both hands to break it open over my knee for the first thousand rounds or so, then it took all of my strength in both arms to open it for the next couple of thousand rounds not using my knee, after that it slowly got better and by ten thousand it was pretty good but not to the point of falling open by itself if you tripped the top lever. Eventually the gun became one of the smoothest opening guns I ever owned but it took probably 20 to 25,000 rounds to get that way.
As Spank said, the key to all this is to keep the gun lubricated and the trunions or hinge pin clean. I always use grease and wipe it out after every outing then relube it. The surface between the forend iron and the receiver must be well lubricated as well, that's where you often see galling on a gun that hasn't been lubed well.
