2x 20-25 min interviews, 2 days apart
Interview 1: personal statement, general discussion; Interview 2: source given beforehand
Practise talking to people about topics of interest
Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
Test taken:
Number of interviews: 2
Skype interview: No
Time between each interview: 2 days
Length of interview 1: 20 minutes; Length of interview 2: 25 minutes
My first interview was about my personal statement and the ideas that I had spoken about there; what kind of reading I was doing; and what other types of history I would be interested in. Then I had some general questions about different types of history.
The second interview was a source based one where I had an hour beforehand to read a source and then 25 minutes of questions on the themes, drawing parallels with other historical periods and picking holes in the historians' arguments.
For the test, I prepared using the practice papers. For the interview, make sure you have read any books you say you have. Have a practice reading extracts / book reviews and check you can discern the arguments. Also just practise talking to people about the topics you are interested in and make sure you can explain why you found them interesting.