Life of any mechanical device is based upon stress, and maintenance. Run a ringed engine slightly rich and no lean runs, it will have a long life. Run it hot and lean a few times and it is toast quick.....less than one hour. Forget after run oil in humid climate, I am in Houston, and you have bearing problems. Do a poor break in such as too rich on an ABC engine.....less life.
You will get a bunch of opinions on engine life hot potato. For me, I get pretty long life from engines...exact hours, I am not sure I can tell you that, but over 15 I can assure you.
If the engines both have the same compression by flipping over with both engines fairly dry and one gets a lot more if oil is added, it is probably worn out. In most cases a rebuild is cheaper than a new engine.
I will go get off my sqeaky soap box and let the next guy take this hot potato.
Good Luck,
Twinman