In fact, it depends on the players. There are a lot of choices so player who likes to think long about his next move will extended the the game.
We played 5 persons, first time with a prototype. We chose the Galaxy medium-sized (14 systems) with the exception that we took versions of track extending game time (Antyimater track is double-sided victory lane has shorter versions of the game and longer version. In long version which we played player win at 12 antimatter at shorter already at 8).
We played about 210 minutes, or 42 minutes per player and ran one round to win (we had to finish ahead of time).
I believe that if we chose the shorter version of the track would end up winning the game with half an hour earlier.
I'd like to chip in on this - though it would probably go as being very biased
I've of course had game go for longer than 40 per player - but that's only when I've done it with new players.. And even with some new players (if they are experienced or easy to teach) we can hit the 40 minute mark.
My experience after the latest Prototype upgrade (1½ month ago) playing time has been very spot on or a bit shorter - in games I've been involved in.