The interview followed a pretty standard process - 2 rounds of technical interviews over the phone (45 minutes each). Recruiters were pretty quick to follow up after each and schedule the next, including the final round interviews.
I've been told that the final round of interviews may vary by candidate but it is generally a full day including 6 interviews. I had 2 30-minute culture fit interviews, 2 45-minute coding interviews, a 45-minute architecture interview, and a 45-minute experience interview. I also had chance to grab lunch with one of the current software engineers to ask additional questions. The whole day was pretty casual and everybody was super friendly.
As far as the culture fit interviews go, I'd heard horror stories about crazy (and even uncomfortable) questions being asked, but this was mostly a fairly standard behavioral interview with a few questions thrown in related to the Airbnb mission.
None of the technical questions I was asked were easy, but I really enjoyed solving the problems and working through the solutions with the interviewers. Definitely brush up on practice problems with Cracking the Coding Interview, HackerRank, and the like. Plus do some research for architectural fundamentals if you haven't had that background.