I just reach the begnning of Act 3. I heard people saying act 3 is supershort. How long will I beat the game (include all the side quests and main quests in act 3) with roach's path?

Depends on how you play. I went through and did everything I could and it took a few hours, though I could see it taking some people more time and others less.

For a point of reference, it felt like it was about half as long as Chapter 2. Granted, a "feeling" isn't very scientific, but it'll have to suffice since there's really no real way to measure the length. If you go in expecting it to be super short, you'll likely be surprised, but if you expect it to be the length of other chapters you'll wind up disappointed.

I bet you're already done with it. Go back and choose iorveth's side for a brand new experience.

i spent 40+hours on this single playthrough(also my first one)just trying to finish every single quests and explore every areas. My equipements are one of the best in game. Suddenly it all came to an end and just can't imagine all the 40 hours gameplay will soon be over. This feeling makes me not want to play... :(