4x 25 min interviews, each a day apart
All interviews: progressively harder maths questions
Do past papers under timed conditions; look at UKMT / Olympiad questions.
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.
Number of interviews: 4
Skype interview: No
Time between each interview: 1 day
Length of interviews: 25 minutes each
Each interview started with a fairly simple maths question, which was then developed into harder questions. It was all fairly relaxed.
I used the past papers online.
I would recommend doing past papers for the admissions test, and especially some under timed conditions. For maths, UKMT challenge / Olympiad questions can also be useful.