How many hours, once the engine is removed from a vehicle, would it take to rebuild an six cylinder engine?
A pro who does this work routinely can do it in “hours”. A novice with no experience will take a few days. If you have all the special tools, reamers, and all the new parts on hand it can go pretty fast.
Its not something to race trough… To do it right can take time… Why?
Taking it apart, cleaning and inspecting parts and putting it back together can be done in a day or less. But there’s more to a quality engine rebuild than that. The engine block has to be honed at a minimum, possibly bored first. That usually means a trip to the machine shop, as most mechanics don’t do that sort of work. If it’s bored, new piston’s will have to be acquired, possibly ordered. The crankshaft may need to be machined, requiring undersized bearings, which will again, need to be ordered. Depending on why the engine is being overhauled, there may be much more machine shop work required as well.
Between the machine shop and the part order delays, a week wouldn’t be unusual.
The time isn’t invested in the actual assembly and disassembly, it’s invested in the testing and inspecting, the machining, the looking up of the replacement parts, the purchasing of materials, etc. Doing a proper rebuild involves the elements Jay mentioned plus lots of checking parts with micrometers and plastigage and such, testing valve springs, reseating valves, etc. etc. etc. A complete engine rebuilder has set up and ready to go such things as cleaning baths, milling machines, boring machines, nondestructive inspection equipment, etc. Doing it at home, you have to spend lots of time setting things up and sending stuff out for work that you can’t do…unles you have a machine shop at home.
What make engine?
Have you ever tackled a problem like this?
There’s rebuilding it and rebuilding it right. The latter is time consuming and while it can vary based on engine type and a number of other factors, flat rate time can often run 25-30 hours on average including the engine R & R from the vehicle. With some engines that are more complex it can run 40 hours give or take.
It’s easy to spend a number of hours just cleaning parts, scraping gaskets, and so on.
Should it be assumed from your post that you’re going to pull the engine and then have someone else build it?
What, exactly are you working on???
A novice with no experience will take a few days
You’re definitely a “glass is half full” kinda guy 
We should all be “glass half full” kinda guys.
But I agree…a “few days” for a novice to rebuild an engine is beyond optimistic.
I’m not a “glass half full” nor “glass half empty” kinda guy - more a “no doubt there’s more beer to fill my glass with somewhere” kinda guy.
Rebuilding an engine for the very first time take a lot longer than a couple of days, I think. If the top end needs machining, that’s days lost. Case check, days lost. etc.