Some years ago someone that worked at Microsoft told me he didn't think any individual engineer who already works on Windows could ever get Windows building by themselves with just the code.
May not take hours, but determined engineer should be able to figure it out.
Just getting the code to compile, link, baking in assets, etc. For a single architecture is a much more reasonable goal.
I'd imagine it'd be on the scale of 1-3months for an engineer to get working full time, but large error bars around this figure
This was done (if I remember right) when governments and big customers had access to the Windows source-code.