My players have pretty much finished it now, in just under 9 months of playing a 3-4 hour session once every two weeks. As noted above, you're not going to (and are not advised to) play through every chapter. Personally I deleted a hook to Argynvostholt, and no-one picked up the Werewolves hook from the beginning. Some parts ran through very quickly e.g. they basically completed Death House in around 3 hours. Same for some other parts - if you have experienced players who follow certain key plot hooks and don't just wander around the sandbox, it can be fairly focused and quick - there's plenty of parts where there's not a lot of combat to slow it down, too.
Personally I did find it challenging to run, but in a good way. Basically there is way too much detail to remember from beginning to end, and the way its arranged makes it easy to miss key interesting stuff if you're reading off the pages at the table (e.g. key plot points scattered at the beginning of a chapter, the end of a chapter, and even in the NPC write-up in the Appendix). So I needed to re-read one or more relevant chapters before each session, add some post-it-notes in places, and keep a simple set of bullet-points to help me in terms of ideas, character hooks/notes, etc. In particular, there are just so much potential for un-scripted events, NPC's galore to play with ongoing, and everything needs to be dynamic according to how your players go about their business which can drive some fun ad-hoc from the DM.
I think you could easily do a good run-through in under 6 months, playing weekly. It's a fun adventure, especially when both players and DM embrace the dark tone of it. As someone who's been a player in the 3.5 version of this adventure, and most of "Out of the Abys", I'd say that Curse of Strahd (5e) is far superior to both, a really good sandbox for players and DM alike. As a DM, I'd say it challenged me to be more creative than a lot of other larger campaigns I've run, but in a good way i.e. adding to the story, not fixing problems.