Depends what you're doing. For example, if my goal in a single player game is to kill all the AIs as fast as possible, it will usually only take 100-150 turns (with many difficult/high or difficult/medium), over two or three sittings, a few hours each. If I want to conquer, usually takes 200-250 turns, and the length of the turns are much longer, since I have to worry about securing captured planets/systems, and redistributing population. The game I'm doing right now is a "ClusterWars", which was popular a while ago, I don't hear is much now though. I am keeping myself isolated until I can research sphereworld tech from low starting tech (high cost, too
), and build one. I got the tech around turn 360, and I warded off AI invasions around turns 175 and 250. It is now around turn 380, and the generator is under construction. I think I've put about 90 hours into it so far, over a month.
So, to put it simply, it depends
Everyone is different, and every game is different.