According to the Steam Community thingy, I finished Episode 2 in about six and a half hours. Meanwhile, Portal -- the game that takes about half as long to play though -- is currently at 15.7. And I'm still playing around with it. I'm going through a
third time now, attempting to get all the cameras for the achievement.
I've finished all the advanced maps and did all the portal challenges (with plenty of room for improvement, mind you), and there's still the timed run and step count challenges which I've barely touched. I've also been watching Youtube videos just to see the neat things people have figured out about certain sections.
I'm fascinated by this game. To call it a "pleasant surprise" would be an understatement