It’s difficult at best to project general answers, as there are so many variables involved. Certainly, engines have a design life. They are generally intended to provide a minimum number of hours for a given application, but this ranges from heavy duty, to continuous service, to light duty. Each manufacturer has different models, with varying design lives, or MTBF (mean time between failure).
We have often thrown numbers around like 1500 hours for traditional gas engines and 5000 hours for diesel engines… between overhauls. These are by no means “real” numbers, they’re just a general rule of thumb we’ve adapted over the years. Your mileage may vary. You could easily double those numbers… if you’re willing to go slow... I’m not.
Keeping a proper maintenance schedule, coupled with staying within the operational parameters the motor was designed for should certainly increase engine life, but there are so many other culprits, it is again… impossible to speculate. Operating conditions and maintenance are the real variable here. No engine will survive without proper maintenance. Overheating just once could ruin an engine, whether it has 20 hours, or 20,000 hours. Failure to maintain the cooling system is probably a more common reason for failure than forgetting to change oil and filters. Another culprit is carbon build-up on the heads and pistons, which is a typical reason for needing an overhaul.
Engine R.P.M. is certainly a consideration. Generally, the higher the RPM’s, the shorter the engine's life will be. Or, the heavier the resistence (load) the more stress is generated. Both will reduce engine life. But pushing the throttle further doesn’t necessarily mean “go-faster”. Displacement boats for example, are a very different scenario. If you have a pure displacement hull, no matter how much more power you throw at it, you won’t go much faster. Any extra horsepower, beyond the speed limitations of the hull is simply wasted.
Within the standard selection of marine diesels, most manufacturers have different ratings for basically the same engine block. For example, let’s take a small, normally aspirated motor rated at 250 horsepower. You can expect to get a full 250 horsepower for long hours. The same basic block with a turbocharger may be rated 350 horsepower. Add an intercooler and some mods, and you’re pushing 400 horsepower. The're all the SAME block, so.. in my futile mind, it’s pretty simple math… the harder you push it, the shorter its life will be. No matter what engine you choose, or comes with the yacht… the life of the engine is directly proportional to the amount the throttle applied. Ask me how I know!!!
Bare in mind, today’s engines are built to much finer tolerances than ever before, hence break in periods have shortened substantially over the years. They will also accept much more abuse and therefore, we can generally expect a much longer life than ever before.
I'm looking for some wood to knock on after that last sentence!