l looked at a boat recently with an inboard 454 Big Block 400 HP with 500 hours on the motor since being put in. Is that a lot? And what would that be equal to in miles approx?

Its not alot. just say the person probably cruised at 20 mph, 20x500=10,000 miles. Its not alot but it all depends on how it was treated and how often the engine was properly maintained

Your question failed to give us one very valuable piece of information & that is the year of the vessel. If she is 5 years old 500 hours could be considered high (100 hrs per season) yet if she is 10 years old it wouldn't. But u stated also ''since being put in.'' Was this a repower? If so why?

jime1991 stated much depends on how she was maintained & that is indeed correct. Oil changes, tune ups, winterization, etc all can extend the life of the engine. Mileage though really does not mean much at all with boating as it is the number of hours on the engine that r of the main concern. The bottom line is past maintenance & how she runs currently. But ask if there is a ships maintenance log that u can look at to help make a decision PLUS have the vessel surveyed by a professional to let u know exactly what u r considering. Money definitely well worth being spent!

Good luck & good boating!!!

I've only seen one marine engine that was worn out in 35 years, most are neglected to death. l want to see used oil in the engine, l worry if it is just been changed. Then it is simple, start the engine and go for a test ride. l like to run it at speed for 15 min or so, and then l closely inspect the mechanical systems afterward. l hope this advise is helpful! Good Luck!!!

Boatin in Va provides a good answer; more important then the number of hours is the way it was maintained during those hours.
l know guys who have boats with thousands of hours on the engines & if the opportunity was there, l would not hesitate to buy that boat. l also know guys with the same type/size boat with 200 hours on the vessel & l would not give them the time of day for their boat..
It is all in how she is taken care of.

That being said, ''ASSUMING'' this boat is a mid-20 is footer ''go-fast 500 hours tends to start to get up there because it is usually the type of boat run at higher RPMS..if this engine is powering a 40 ft trawler (unlikely) 500 low RPM hours is not much.

P.S..Comparing miles to hours is ''apples to oranges''.their is not a comparison.