It was a one-shot interview for about 45 minutes - 1 hour, where they asked interviewee to prepare a case study beforehand and present them the result. The preparation time was ~3 hours (it was more than that in reality). Interview began with friendly introduction then the case study. The interviewer were asking pretty technical questions related to the case study, more about how we came up with the approach. It was relatively average difficulty for experienced candidates, but could be somewhat hard for those without prior experience.