I remember one of the developers (I think Josh) saying they were developing Icewind Dale and they thought it would be a 30 hour game when actually it was like 80+ hours. That's why he said he doesn't like giving a concrete number because it'll never be accurate.
Especially with a game of this scope. Very FEW games come close to the length, breadth, scope, etc. of Infinity Engine games. With bigger games, you have to extrapolate on a game length if you really want to give some sort of finite number. But in reality, this isn't really the best choice.
To the OP, I wouldn't be worried about the game length. Josh said Baldur's Gate 2 was....MASSIVE. And that PoE won't be BG2 "massive" but be "comfortably" within range of it's length. In other words, between BG1 and BG2.
Keep in mind, they will hopefully give us an expansion and/or mods will extend the game's length. Replaying the game doesn't hurt either.