Mine is an ATK reman, and they say to change it at 600 miles, my mechanic neighbor mentioned 500 miles. I'll probably go with 600.
Drove the truck about 55 miles yesterday and it still wasn't idling right (died five or six times at stoplights and such). Took it down to the neighbor's and he scanned it. Came up with like four or six codes (same codes on both sides) basically saying that the O2 sensors were reading it was too lean. Looked around and found a guy who had the same problem and it was a vacuum leak at the back where the lines go in at the bottom of the intake manifold. I took my truck home and sprayed some starting fluid back there (yes I know, starting fluid around a running engine isn't a great idea!) the truck rev'ed up a little, so I knew I had a leak.
My brakes had been really stiff since the rebuild, and not that great, I wasn't really sure why, but I guess I should have made the connection. Ended up taking the intake manifold off (alternator, intake, throttle body, misc vacuum lines to the top of the intake manifold), and finding that the vacuum line to brakes was not connected. Hooked it up (a huge feat in itself, not easy to get the clamp on!) and put the truck back together and she runs/idles GREAT! I'll run her down to the neighbor's tomorrow, and have him scan it again to make sure we are good.