Hello Georg, [...]
I'll grant it's a hard job, but look at WW-II, throw in some large ugly unifying force and Stuff Gets Done! Heck, look at 1900-2009. I'd say that most of the tech that existed in 1900 could be built from the ground up in under 50 years if the people didn't needed to do any R&D and are motivated enough. As for some hard numbers, I recall a NOVA show where a construction planner was asked to set up a time line for the pyramids using period tech. The time line was under 3 years (2.5 IIRC).
With the best assumptions you can reasonably expect to get I think the timeline would surprise most everyone.
