Games can take as long as you want them to, anything from an hour to what will probably be literally weeks of in-game time.
I played the largest 'small' map, 27 planets on one star in 6 hours, the smallest small map, 7 planets, 1 star, in 45 minutes, the largest random map, 5 stars, and 80+ planets, I believe, in 19 hours, and I'm starting a 10-star, 400+ planet map from the game's in-game custom map making tool which I expect will take hundreds of hours, especially since I have it set to slow. Outside of the dead-end, one-exit 3-planet cluster my starting world was in, the phase jump out to the next planet was 45 minutes long, for my scout. needless to say, I'm 8 hours into the game, have 8 planets colonized, and I have the tech tree fully researched out, and the fleet is now building up. thank god for windowed mode and dual monitors, run Sins on the widescreen, and pull up a browser on the side, makes for a great way to pass the time.