Just a hypothetical question:
What if a couple of architects, young, were able to draw up plans for a city the size of, say, Dallas TX (metroplex and surrounding vicinity). They are given full support and funding (they're pretty rich themselves) and they oversee the unceasing raising of dozens of mid-sized skyscrapers and surrounding neighborhoods. How long, realistically, would it take for them to build the city and actually see it prosper? Would they be pretty old by the time the city becomes an actual city? 20 years? 50 years?
Basically, if building a city were a designated "project", and not just something that naturally develops over time with increasing population.
Alternatively, how long would it take for them to "gentrify" an existing city, the same size, and modernize it, knock down old buildings and build new ones, etc...?