A friend says "1000 hours" as a general rule for the lifespan of a gas mercruiser inboard/sterndrive engine. I understand what you say though.
If compression is good, oil looks fine, and a mechanic can't find anything immediately wrong though, what I'm after is a ballpark lifespan for an older boat if I continue to care for her properly. (and a ballpark to use as I do some preliminary looking at ads for used boats)
For example, one boat I'm looking at has 200 hours on the mercs. Another less expensive boat has 600 hours on the engines, but the owner assures me they run great, have no problems, and they look spotless indicating they've been maintained by someone who cares. If I plan to put 100 hours on the boat I get per season, what I'm looking for is some ballpark idea so I can figure how much more are the engines with 200 hours worth to me than the engines with 600 hours already? (i.e. in an average case if they're in good shape now, how long will they live for most people before major money is required to rebuild or replace?)