Depending on where you are in the game, a turn can be as long as 50 years or as short as 1 year. So it is not by any stretch of the imagination that a leader could finish any project within 1 turn. As the farther into the game you get, the more advanced you get and the fewer years inbetween turns. Even the massive projects we start in our time can be finished in a year or 2 once the planning stage is completed.