Regardless of the car, overhaul of the shortblock, or really any other portion of the engine is not dictated by mileage, but condition. There are three main indicators of this, oil pressure(or lack thereof), oil consumption and compression.
Something may happen that makes this necessary at 4,000 miles, or 400,000 miles. Mileage will not be the indicator to go by. Use of the engine, maintenance and luck all have to do with when this will be necessary.
The head had to come off my M103 engine at about 170,000 miles, there was absolutely no ridge on the cylinder wall and the cross hatch was still prominent. I'm sure there have been other M103 engines that were completely worn out by 50,000 miles. It all depends.
Best of luck with yours,