Monarch level. Standard map size, continents, everything standard. How fast should a good player be able to win?
In real time? Like, play the game as if it was a RTS?
Or counted in number of turns? Regardless of how much real time it took.
For the later, you can check the hall of fame here at CivFanatics for an answer.
checking for earliest win on monarch.
Also, how much do you develop your cities with improvements, etc.?
Only cities that will pump settlers for an extended period of time will get a granary.
Only cities that will be pumping military units will get a barracks.
Only core and semi core cities will get a market place and a library eventually.
Only half-corrupt (semi-core) cities will get a courthouse.
Only core and semi-core cities get an aquaduct and then only when there is enough food around for it to grow past size 6.
Only after a lot of considerations will some of my core/semi-core cities get a hospital and or a factory. Largely depending on whether this would increase the production enough to reduce the turns needed to produce a unit. without compromising neighbouring cities.
All core and semi core cities that are coastal, and where some citizens work on the water, will get a harbour eventually.
Only when I try a 100K culture victory will I build other cultural structures, and then only after I conquered a large amount of land, and stop further conquest at that point.
Cities that are outside of my core and semi-core area will be turned into specialist farms and will get no city improvements at all.
When to build a structure, if it is build at all, is just as important a question.
(though I sometimes play a variant game where I don't follow these rules.)