The oil filters are always a pain although I have a pretty good system, it can still get messy.
As long as you are doing the oil, you should do all the fuel filters.
All together, including about 15 minutes to warm up the engines, clean up, tool clean up, etc. I would guess the oil and all filters changed will take four or five hours, start to finish.
I work slow and take breaks, I'm not sure if the marina mechanic does or not. I do know he is on the phone a LOT, do I get charged for that??
I'm doing slow, too. This time, partly 'cause I'm learning. Partly 'cause I'm taking notes, developing a procedural work flow and checklist for nest time, etc. Partly due to heat, both engine heat and ambient temps (high 80s yesterday). Partly 'cause I'm old.
We don't have spin-on oil filters; it's paper filters within drainable canisters... and there's a way to not make much of a mess. Ideally that includes an overnight drain. Took me a bit to find that way.
And then it also took me a bit to learn how to work with (loosen) bolts using copper crush sealing washers.
I don't think that twins would actually double the time. I think I've spent more time getting myself into position to do something... that it took to actually do the something once I figured it out.
Looks like Day 1 is warm up and drain, leave the ER due to heat. Day 2 is beginning the filters drain. Day 3 is replacing filters, and refilling engines. Probably no more than 2 hours actual work time on each of those days, including recycling. (Guessing, but now that I have more of a clue.)
But I reckon next time, I could actually do that 3-day oil change in 3 days.
Our fuel filter changes are on a different schedule, so far, and it happens we changed the secondaries last month... when I learned how to do that, too. And our genset and gears are on different schedules, too, so I'm only doing mains just now.
I'd guess that a generic marina mechanic -- who may or may not be familiar with a given engine -- might need phone support from time to time. Or maybe arranging lunch with his wifey, whatever.
-Chris