Rebuilt by whom?  The warranty on the engine will probably have some
specific requirements, so be sure to find those out from the rebuilder
themselves.  Also, there's really no such thing as "any generic
engine".  Different engines require different procedures, based upon
whether the block or head is made of aluminum or iron.  If you've
rebuilt it yourself, the rebuild kit instructions might mention it.

Has the engine been run for the first time yet?  Usually this involves
setting the idle up around 1500RPM and letting it run with no load for
an hour or so (depending).  But this might be different for your
engine.  Usually you want to change the oil at the first 1000 miles,
but check with the rebuilder/remanufacturer.  They may insist that you
have them change the oil (so they can inspect it).

Once again- take all this with a grain of salt, but get documentation
from the rebuilder.  They will have a very specific procedure for all
of this, along with things like head bolt retorqing intervals, valve
adjustments and the like, if necessary.

-phaeton

I, perhaps singularly, always recommend frequent oil changes.

At first, do not over stress the engine.  Drive it for a few hundred miles
near home and at reasonable
speeds and accelerations just in case something wimps out on you.

After 200-500 miles, approximately, you can probably rest assured that the
engine was put together
reasonably, and that the likelihood for a sudden failure is decreasingly
small.

Everything depends upon the quality of the machine work, parts, and rebuild
technique.  A rebuilt
engine can be as good as, or better than, the original, but some arent.

Give it a calm and decent breakin, and take care of it .. I wish you well.