How many acts there will be and some more

According to the READ ME article in the general forum 3 so far more to be added!

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I think Blizzard hit the spot with D2. 5 Acts, all of excellent quality, all of different design, and perfect length.

Too bad the best act was the shortest one :/

I was surprised by how short the acts were. Me and my friends finished the two ones currently available in a day, which was a bit of a letdown.

The third act will come with the open beta afaik and you won't be that fast on the higher difficulties because it really gets much harder :)
and well, hack & slashs usually don't last that long, it's more about grinding the same content over and over again ^^
just as you could finish diablo 2 (first difficulty) in 1-2 days aswell