If I ever get around to going back and finishing it, my current 10-planet game will probably be my shortest by a decent amount. I think I was on track to win in 3-4 hours total, which is nothing special, but considering how turtly I tend to be, that's fast for me.
My longest so far is 27 hours, and I didn't even win that one. If I hadn't screwed up where I did, I probably would've won and gotten the 36 hour achievement. It did take a while, because I don't fast forward much like some people, but I also rarely pause, so I guess it could've been worse. I can't imagine what a game of that length would be like for one of those people who spends 90% of their time paused obsessing over every little detail. I'd like to be able to fit more than one or two games in per year, after all. Heh.