Any rule of thumb? __________________
It is claimed that the usual boat does 200 hours a year. With a 10 year old boat 2,000 would not be uncommon .
A dock Queen might have 1/4 that time , so would be very suspect that the REQUIRED maint was done on time.
A well maintained engine of course will last its full service life easier than a "Run it till it Dies" program.
A good look in the engine space might show the level of maint.Proper oil, antifreez and filters.
Diesels can run extreme hours , but seldom do in a boat.
Figure 5000- hours or so on a small diesel (taxi or farm equipment origin) and double that with a more industrial engine source.
Old Gas engines will usually last 2000 hours IF fresh water cooled , 7 years or so if seawater cooled.
As maint and use pattern is more important than hours its hard to guess how much life is left in the old bucket.