I sold this 1975 Delta 2 years ago. My parents bought it new, and I then bought it from them several years ago. The motor pictured above is a 302 Ford. That thing ran extremely well and never had any internal maintenance done. It was extremely well kept, but I wouldn't use the term "babied". The boat was constantly garaged except for trips where it was in use daily. The boat was always professionally winterized/inspected every year. So it was well kept. Our normal cruising speed was ~2800 rpms, which was ~25 mph in that boat. Occassionally we would cruise at higher rpms such as during trips with other boats that were generally bigger and had faster cruising speeds. A few times a year we would run her wide open for a minute or two to "blow the cobwebs out". But that boat was a great skiing boat. It taught several people in this area how to water ski, which isn't the best thing for an engine given the constant throttle up / throttle down in those situations. Plus, for the more advanced skier, we would pull along at ~3300 rpms (~35 mph) for extended periods. So once again, extremely well kept but not babied.
I saw my old boat in a gas station a few weeks ago and pulled in to say "hi". The current owner said she's still running like a top. That was a happy day, and I hope to run into that boat for many more years to come.
So to answer your question, that engine (as well as any other engine) will last as long as you want it to assuming you're willing to put the time and/or dollars into to properly maintaining it and don't run it into the ground. I would recommend asking questions about getting maintenance documentation, how the boat has been stored, how the boat was used, etc. to get a general idea of the condition of the engine.