[stugiv]

Its hard to answer exactly. As sailor2 says, boats that run a lot can get several thousands of hours out of them. For pleasure boaters that put less than 100 hrs/year on the engines, I wonder if years isn't a better measure. When the engine isn't running, corrosion is happening somewhere shortening the life rather than wear on bearings or piston rings from use. For low usage boaters, I would think at least 20 years if properly maintained.