I think you're being naive. Microsoft has spent 1000s of engineering years on their build system. You aren't going to just replicate that in a week.
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.