These days we follow the lead of Bob Hoover in starting up rebuilt engines. If the engine has new rings, the first 15 minutes of service are occupied with running the RPMs up and then letting the throttle slam shut. The engine is always running at different speeds and never idling. Of course, someone is also nervously lurking about, sometimes using a listening pipe on the crankcase, hoping not to hear untoward sounds.
If just the bottom end has been done, we still do it the Bob Donald's (Boston Engine Exchange) method of 15 minutes at 2000 RPM. It makes the job of listening easier, too.
In either case, the oil gets drained and examined when the initial run is finished.
A rebuilt engine should give about 2000 hours of service.
That's about 100,000 miles. Whenever someone tries to sell me a car
with a rebuilt engine, I ask "who rebuilt it?". It makes a difference.