My group can get a game of 4th edition finished in four to five hours. We are seasoned players, though. Twilight Imperium is both a strategically and tactically rich 4X game, which is why it's one of my favorites.
That said, I am not opposed to long games. I recently played Fire in the Sky, which took me and my opponent 4 four-hour sessions to complete.
I look at it as no different than a campaign game such as Gloomhaven. Gloomhaven took my group two years to finish.