After 8 hours I pulled the mounting screws. It took a light tap with a hammer, but broke free easily. The surface is smooth and free of voids.
Reinstalled the screws and will let it cure out for another 12-15 hours.
It looks like my Burris XTR low rings are just a hair too low for my old scope's 50mm objective. I may wait to see how my new scope fits before sending these back. I would prefer it to be as close to the bore as possible.
Is ring lapping absolutely necessary?