$500 sounds ok. If the guy is good it's about 1-2 hours but it takes a educated feel to set the clearance.
$500 is fine IF he has done it many times.
An hour ot two is impossible as the proper procedure calls for the head torque to be checked and each valve adjusted also, on a warm engine.
A really good experienced fellow will need a half day , IF the engine can easily be turned with a long bar.
Da Book sez to adjust valve on cylinder (say) 4 when cylinder 1 valve is in a certain position.
This requires great care, but the engine should sound like a sewing machine if all is done with skill.
Da Book to search for is "Students Manual Gray Marine Diesels"
first printed 1942 , I have the 3rd edition.
It is spiral bound and mine was about $75 on line, in fair condition.
Amazon shows one for $10.00
Best $10. you will ever spend.