http://pixelvulture.com/2015/09/crystal-dynamics-lead-designer-mike-brinker-chats-about-rise-of-the-tomb-raider/

What about the size of the game? how many days off should I take to get this one done?

Expect to spend around 30 -40 hours getting everything done. This is gathering every weapons, ability and completing every side quest. It all comes down to how you want to play through the game. Running through the game will take you around 15 – 20 hours. So expect double the play time collecting everything and exploring every secret tomb. In terms of scale, all our hub spaces, the areas that allow you to explore and choose your own way to play, have been increased by three time the size. So there will be lots more to see and do in each area of the game too. We have more challenge tombs, crypts, caves, and a bunch of extra things to do like the side missions for the Remnant people, and the tomb guardians.

If I have to give an opinion, maybe the story can stretch itself to the 15 hours mark, but I don't believe at all that it will take 30 or 40 hours to complete all the side missions. We are not talking about an RPG after all.

TR13 was a solid 12 hours for me, but only took a couple more to get around 100% complete. Good to see this game essential twice as long, which I think is very important for an SP only game.

I personally find sub 6-7 hour campaigns inexcussable. Lookging forward to Halo 5 which is also supposed to be more than 15 hours long. Hope Gears 4 is the same. 

Uncharted 4 should be this long, being the ''last'' in the series and how TLOU was nearly 15 hours long. 

Until somebody speed runs it in a couple of hours then everybody will claim it's short. That what usually happens anyway.

15 hours seems enough. I never really cared about the "100% completion time", very few titles interest me that much (I prefer to start an other game). But too bad there isn't a MP mode, I've found the first one a little short because of that (finished it more or less in a day, never played it again since :/). I have maybe hundreds of hours on TLOU's MP, it really increases the value of the game to me.

so about the same as the last game which was a good campaign.

Wow pretty impressive. Can't wait until the Ps4 version comes out next year to play it.