tl;dr: the adventure can be very easily stripped back, and is designed that way. I'd say about 20 sessions would be sufficient to do just about everything in the book.
I have been running since August, about 13 sessions I think; we tend to play for around 3 hours. My players have done a body blow to the bulk of the plot, but other posters are correct in that the sandboxy element makes this hard to judge. I've let the players do whatever they wanted, with only the Werewolf Den (cool as it is) being left out as there was no hook for them to learn about it. In those 13 weeks, they've done:
- Death House (Appendix B)
- Barovia Village
- Old Bonegrinder
- Vallaki (mostly, they left it under the control of Lady Wachter and they're not happy with what they hear is going on there now)
- Van Richten's Tower
- Wizard of Wines (they went here to get access to Krezk, but after asking for Ireena to be let in they never bothered entering themselves, not liking what they'd heard of the Abbey, and thinking it was probably as good an idea as any to leave Ireena here)
- Yester Hill
- Berez (bastard tough but quick, like most places in Barovia...)
- Tsolenka Pass
They are about to enter the Amber Temple, which I expect to be a 3-week effort; they got an invitation to the Castle this (ingame) morning, and they also want to help Ezmerelda rescue Van Richten from the dungeons of the Castle, and so they (rather unwisely, in my opinion) set the plan to march from Wizard of Wines to the Amber Temple, examine the model of the castle that they know can be found there, then march to the Castle and distract Strahd over dinner while Ezmerelda does her thing - all in the same day.
This kind of shows the difficulty of estimating the length of the campaign - this sequence of events is really determined by what my players have done prior, and what they want to do now. The plot is wide-open for this one, even if all roads end with a fight against Strahd somewhere in the Castle. However, you can easily construct a more rigid plotline, and it is also easy to remove stuff. For example, Amber Temple is easy to remove; you could also slice out Van Richten's tower and the Werewolf Den without causing too many seams to appear, and even stuff like Old Bonegrinder is totally optional. Most chapters take a week or two to resolve, with the exception of the Castle, Vallaki, and Amber Temple. The 'absolute mandatory' elements of the plot are:
1) Arrive in Ravenloft
2) Find out about Strahd and his backstory
3) Enter the Castle
4) Kill Strahd
50% of the book is used to have (very interesting and flavourful) means of gaining power and information between points 2 and 3. I'd also say that Vallaki and its politics are near-vital to the plot, since it can drive a lot of what happens next, but again it doesn't
need to happen.
I once suggested to take a one week holiday together with some friends to play a whole adventure in one week, but my friends didnt like that plan
I once heard of a group that did this, only they rented a lighthouse and played Call of Cthulhu. Sounds glorious.
