That's about right. Our total played time for Normal was about 9-10 hours total, including exploring the entire map. Quite disappointing actually, I do wish Blizzard had been a little more creative than just expecting players to repeat content for superior loot.
With that said, Nightmare is quite a step up from Normal and actually gives players a challenge to think before they leroy an entire room of mobs. Honestly though, for my friends and I, we'll probably be stopping at the end of Nightmare. There's really no entertainment from beating the game 3 times in a row, even if you get shinier gear to equip. It becomes extremely repetitive after that point, as seen with D2. Had Blizzard made the game into a true "progressive" difficulty, in essence, where there lies a compaign for each set difficulty, I do believe the game would have been that much more enjoyable.
All in all, would I recommend spending $60.00 on this game? If you're a D2 fan, no, you will be disappointed as D3 is a mediocre, semi-revamped D2 with half the content and the loot is so numb down that six year old's can theorycraft the equipment.
However, if you're new to the Diablo franchise, I'd consider it; I mean, you'll be entertained but once again, the game does become extremely limited, extremely fast (in retrospect to a "campaign" game).
Back to League of Legends in the meantime.