This is a re-post I made earlier in the week.
It actually scares me that 1/3 of Act 1 takes roughly an hour to do. That seems like a lot less content than the first act in Diablo 2. I swear the first act, especially while soloing, took around 5 hrs, even more if you wanted to explore every cave/dungeon (keep in mind this isn't considering a speed run, when you know where to go and what to do constantly).
Since it takes roughly 3 hours to do Act 1, and each act gets shorter and shorter, we'll be looking at around 10 hours of gameplay to beat it on normal. I know this is just speculation, but 1 hours for 1/3 of Act 1 seems very short to me.
I'm hoping that the next portions of Act 1 take longer. From start to finish (on normal) I think the game should be roughly 15 hrs minimum. I'm drawing a comparison to other games - most games of a linear nature take < 20 hours.