Outboard Motor Life Span?

I am currently looking a purchasing a used sportfishing boat powered by 2 250Hp Yamaha fuel injected outboards. The engines are 99' models and have 500 hrs on them. Physcial appearance is great and they run good. I plan to do a compression test. My question is how much longer on average can I expect these engines to last? I would imagine that many of the hours have been accumulated while pulling baits.
Thanks

 

Pulling baits are hard hours on any engine, low speeds and temperatures lead to carbon buildup, and scoring of the cylinders. most outboards are good for 1000-1200 hours, longer with exceptional care. My '96 175 Johnson is still going strong, though it now belongs to a friend. It has at least 1200 hours on it, and has had only the starter and power pack replaced. I used good oil and flushed the engine after every use. It all depends on how good the previous owner maintained them. If they look good as you said, and have been used in salt water, chances are the previous owner took good care of them. Good Luck

 

About ten years in salt water is the rule of thumb. Corrosion will end the engine's life just as running.