How long does it take to build a wall? has been closed as being off-topic as a real-world research question (per campaign research questions, II).
I agree that real-world research could answer this. I disagree, though, that it's the only way to answer that question: I believe that answers that do depend on RPG expertise exist.
For example, the answer I'd have written:
5e doesn't have rules that cover this sort of thing. But fear not! You're in luck! (I go on with my usual praises for 2e's Castle Guide, provide examples of how well 2e plays with 5e.)
So then I'd detail how I arrive at a production multiplier of ~2/3, factor in overhead costs, blah blah blah, and it's \$X\$ weeks (and about \$Y\$ gp).
This would be an inspiring answer, clearly based in my expertise sitting alone in a room with my RPG books, designing towns and structures in far too much detail.
I think the question should be reopened. But I think it worth laying out before my vote overrides those of other 5e gold-badge-holders and another diamond. Please express 'yea' or 'nay' as you see fit.