3 times. Once for both main paths after the game was released and after EE-patch I finished Iorveth-path for a second time on dark mode. I also loaded the "at the crossroads" save to do EE with Roche (everything at prologue and first chapter were the way I'd want them to be already) to do a partial fourth playthrough.
Anyway, a piece of somewhat related criticism: I preferred the TW1-way of subtle consequence where it just kicks you in the balls and gives thing to reflect upon ("Did you really choose the lesser evil?", "That's the price of neutrality" kind of stuff) instead of changing an entire chapter. Overall, I believe consequence in RPGs should exist to hide plot holes that would occur as a result of otherwise linear narrative and to give things to reflect upon.
Significant differences are nice and all, but if you look at how much the game references your past actions, there aren't all that many of them. It's not like TW2 does poorly on that aspect (hell, even a game as renowned as Planescape: Torment falls short of it), but since the length of the game also suffered and games like TW1 or PS:T work just fine with lesser amount of gameplay-consequence, I think the price was too heavy to pay (overall lifetime doesn't change but one playthrough is less satisfying and it distracts you from more subtle consequences). In addition, the game makes it damn clear that *this* is the decision that influences the later outcome: that, I think, undermines the playing-in-character experience because you'll be more inclined to metagame your decision (similar to color-coded dialogue options in a certain other game and unlike TW1 where outcomes are unexpected and correct answers don't exist).