First, a person will contact you to arrange a time for the interview. For internships, I had two 1-hour phone interviews on the same day. It's better to have at least two weeks to prepare after you get the contact from Google. Also, the phone signal might be bad, so be prepared to have an alternative way to contact. The first question is about bitwise operations. Studying from "Cracking the Coding Interview" might help.

Design a class where you can add elements and return the mean of the latest N elements.