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: None
Number of interviews: 2
Skype interview: no
Time between interviews: not sure!
Length of first interview: 20 minutes; Length of second interview: 20 minutes
One of my interviews was a 'maths' interview, and the other was physics and chemistry. We could express a preference beforehand for this one, so I said I would prefer to talk about chemistry. I found in the chemistry one the interviewers were really friendly in helping me through the problems whereas in the maths one
The first maths question I was asked I had seen before so answered very quickly. The interviewers could clearly tell this and moved on to things I had never seen very quickly, so they just want to see how you think, not how much interview practice you have had!
I prepared by talking over old test papers (mostly Oxford PAT) with
Interviewers are just looking to see how you think, not necessarily what you know! So I'd say it's good to explain every thought you have as it shows them your problem solving skills. If I did it again, I would be less worried about saying stupid things and just explain clearly how I reached every conclusion.