When the frame rails are properly peened to tighten up the slide/frame fit, does anyone know how long this procedure keeps the pistol tight? 100's of rounds or 1000's of rounds?
Depending on how good the work was, 1000's of rounds.
I've got a pair of early 91A1 Colts that are approaching 75,000 rounds since they were peened down at the 75,000 round mark. They haven't changed. It depends a lot on how the barrel is fitted. If it's so tight vertically that it forces the slide upward against the frame rails, it won't last as long... but that holds true for any slide/frame fit... peened and refitted or not. I know that it's all the rage to fit barrels uber-tight in the quest for ragged bughole groups at 50 yards, but I don't do that. It's fine on a Bullseye pistol, but for a general purpose shooter and carry gun, it's not necessary and can be counterproductive.
I've had one gun peened, and after then spending about five hours fitting a Kart EZ-Fit barrel, it was "tight" and would shoot five-round cloverleafs at fifteen yards (with me doing the shooting!!). Within 2000 rounds it was noticeably looser, and the edge was gone off the accuracy. I now have about 10,000 rounds through it, and the fit doesn't seem to have deteriorated any further.