When you idle the engine, the wear to consume one gallon of fuel is equal to driving roughly 30 miles. That's for a medium duty engine. It's slightly less miles (20-25) for a heavy duty engine, slightly more (35-40) for light dirty engines in cars and pickups.
Some truck fleets measure engine hours rather than mileage for maintenance purposes. It just depends on how they are operated. City driving, with frequent stops and lots of idling, those fleets tend to use hours.
For boats, they don't have odometers on boats, but they do have hourmeters. A typical gas engine on a boat will run 1500 before it requires a major overhaul (twice that for a car). That's under typical conditions of salt air, damp bilges, intermittent operation, neglect. A diesel engine under the same conditions will last 3000 hours. If anally maintained, gas and diesel boat engines will last 5000 hours and 8000 hours, respectively. Since the typical boat owner runs their boat engine about 200 hours a year, they can go many years between overhauls, especially with a diesel. These are not outboard motors on a bass boat, I'm talking Pontiac long block V-8 engines and Cummins diesels.
I've owned a boat or two and that's when I learned about hours v mileage, how it all works.