Exhaust manifolds

It depends entirely on how corroded the bolts are that hold it on the engine. I would say 4-6 hours if it goes bad, 2 hours if everything goes right. Are they painting the parts before install?

Make of engines and brand of exhaust manifolds has as much to do with it as does access. Changing the 2 outside bolts to studs, helps save time CONSIDERABLY in hanging new exhaust manifolds on.

Crusaders & Volvo Penta don't seem to have a lot of stuff hanging of of them. Mercruiser loves to hang hoses all over their beast.
LBCs have 2 bolts per exhaust port way under the log.

We did a manifold job on a Carver/Volvo 8.1 couple of months ago. The outside log had 4 of the 8mm mini studs fail.
It may have been easier to pull the head but we did get the studs out. I laid over the engine, worked upside down with my head or shoulder against the fuel tank, hand drilled and heli coiled every one.

We had planned on both engines in one day. The Port engine ate the whole day and we charged 6 hours for one engine. With a helper that 6 hours was really over 10. It could have been next day re-assembly if not for a second person.

We try to be fair with our customers. Not to say your shop would not be fair but I should of charged that 10 man-hours.

I bring this sad story up to answer your question the long way; THERE IS NO EASY ANSWER TO YOUR QUESTION.

The Stb engine is planed just before foots-bal season.

I have done Crusader manifolds in a couple of hours, but on the exact same boat it has taken days.
The key is how long it takes to disassemble the manifold and bolts.
the new one goes on pretty quick.