Mansa and Hiawatha are under a month each straight through (23 days or less, depending on the order you do them in). Moctezuma is well under two months, Amelia and Haile similar but a bit longer. Those three tend to be resource constrained vs time constrained, particularly haile and amelia.
Leo is 734 days before discounts. 570 with mansa and hiawatha done. And gets down to 497 with atomium. Since you won't wait until atomium and atomic age to start it, but you might as well finish mansa and hiawatha first (fewer people as well as less time), 18 months is a fair best case estimate. And that is a lot of people. Leo is a worst case, since the last layer is oil, and takes 7 people for each stage, starting at 5 days and creeping up from there. Those layers don't get a 30% time discount.
Saladin starts even higher at 794, but drops further with no oil levels.555 or 476 with atomium.
You can lower the time by finishing leo, or the worker count by finishing sejong, but they are so long already that you don't want to wait for the offensive boosts. Well over half of the total times is either top levels that aren't that useful, or the final few stages of low value areas. The first 5 levels take less than 1/3 of the total time for any of the lower steps.