My interview was around 15minutes. Came with my wife as I still had a pending I-751 ROC, but the IO only asked her like 2 simple questions.
No problems or issues at all. All positive and professional. I was approved on the spot and the IO told me it may take around 6 weeks to get scheduled for oath. Now I know it took nearly 8 weeks. The IO said its just all system-based, not manual, so I think once there's a large enough group, the system schedules them all together. So depending on when your interview is, you may wait longer for more people or wait short if they already have a large group. But that's just my guess based on what I was told.
As for civics prep - I learned from the official mobile app (USCIS: Civics Test Study Tools) on iOS. I just kept taking a few quizzes daily for a week or two before the interview.
As for English prep - I only reviewed some vocabulary for writing test (mainly spelling check) from here https://www.uscis.gov/sites/default/files/document/guides/M-1122.pdf
Good luck!