I did Brinewall in two 8-hour sessions, and Ravenscraeg in another two 8-hour sessions. Other groups might take it faster, but I have a RP heavy group.
Brinewall took my group about 10 hours in total. Though they did end up having to retreat and attack again due to one foolhardy character getting himself killed, so they probably could have managed it in a bit less time.
My group started 2 months ago and we started Book 2 last Friday. Book 1 Parts 1-2 took 2 sessions and Part 3 took 2 sessions (even took down a CR 7-ish encounter with no casualties, beating 4 named NPCs at once which may have shortened it). Our games run from 8-12 hours long, though, so YMMV.