There really isn't one. It all depends on the boat and engine package. If you equate a car needing an engine rebuilt at 100,000 miles, the Merc 1075's need to be done at around 100hrs, and on the low end the 502's need to be rebuilt at around 500-600 hrs. But it all depends on the driver and boat. There is alot more torque at 3500 RPM in a boat motor than rolling down the highway at 70 MPH.