Here's what I have:
2004 8.1 Volvo Penta gas engines with heat exchangers. Engines have about 1,050 hours on them, 800 of which were fresh water until around October 2022 when I started down the east coast ICW from NY to Florida, so maybe 250 hours in salt water. Engines run rock solid, no issues.

The conversation/concerns:
Another 356 in my marina just blew an engine. Topic of conversation is "when have the manifolds and risers/elbows been replaced?" For my boat, never. From the outside all manifolds/risers look great. Several people are telling me it's not an "engine hour" item of concern but a "calendar" item. Stating all manifolds and risers should be replaced every 3-5 years max regardless of run time. I've done some research online, e.g., BoatTest article states 3-5 yrs as well (or I'm just asking for a blown engine repair). I don't have the expertise to take on a job this size and would need to have a mechanic do it. A quick quote was offered of $5,500-6,000. I know it's a lot cheaper than a blown engine, however, if I can hold off for another year or so while in salt water I'd do it.
What is your personal experience with replacing manifolds/risers on a VOLVO gas engine with heat exchangers, etc.? What condition was your manifolds in when you pulled them? Did you think, wow, they look great. Could have gone another 5 years? Or was it more like, damn, got to them just in time. What was your time frame, e.g., 3 yrs and replaced? 20 yrs? Or like my boat, never.

Thanks in advance for all your input.
Bob