I'm running a pair of 87' 454 mags with 650 hrs. not using any oil or making any noises. anybody have any idea when I should consider a rebuild?

Hours are not a great indicator on the need for a rebuild in many cases. as long as they've been well maintained and not abused, keep running them until they start using oil or loose some power. If you really want to find out how healthy they are, then do leak down tests.

Leak down test is good. Pull a valve cover and have a couple of your springs tested for strength.

If you don't find anything, leave them alone.

Murphy's law says that if you are thinking of having them rebuilt, the next time you run them they will break.

I know a guy though that has over 1500 hours on one of those engines.

When they start making noises or stop running altogether

I've got 1161hrs on my '91 454MAGs, in saltwater no less. They'll still run right up to 5k rpm but we're way past when most would at least change valve springs...and rockers since we're in there. But unless you are chomping at the bit, and the oil pressure is still good, leave 'em be.

I'm dying to go through mine with at least a complete valvetrain (cam, lifters, pushrods, springs & rockers) but down here in FL, I don't want to risk taking the boat out of service if something doesn't go right.

had a compression test done about 150-200 hrs ago and all was well, I just didnt want run them to long and do any damage although I am considering some mild performance upgrades