3x 30 min interviews, about half a day apart
Interview 1: Two problem questions chosen in advance; Interviews 2 & 3: problem sheets
Practice questions and short videos
Start early with researching your subject
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: 3
Time between interviews: 6-12 hours
Length of interviews: 30 mins
Online interview: No
All the questions were specifically maths questions (i.e. no personal statement questions etc). When I arrived, I was given a problem sheet with 6 problems, from which I was to choose 2 to attempt (I chose a question related to summation and integration, and a question just related to integration), and take those to my first interview. At that interview I discussed my solutions to those problems, after which I was given extension problems related to the questions I chose.
At my second interview, I was given two series of questions (one was on graph sketching and differentiation, and the other was on trigonometry and integration). The interviewers asked several questions, eventually leading up to a larger question which was then more answerable due to the previous questions asked.
My third interview was of the same style to my second. One thing to note is that I had
Specifically for interviews, I found many practice questions online, and attempted to work through as many as I could. If there was a question from there that I couldn't complete, then I would try and research the topic to gain a better understanding of that question, and then would re-attempt to answer it.
More generally, from the start of year 12, I enjoyed watching short youtube videos on random different maths questions (particularly on integration), which made me much more capable of dealing with questions outside and beyond the A level specification.
Only practice papers
Everyone says that if you felt like your interviews went bad, then they probably went well. However, this is not to say that if your interviews go well, that they probably went bad, so don't be disheartened if the interview felt like it went well.
Also, I would say to start preparation very early. You should love your subject, so it shouldn't feel like work to research about your subject regularly, so do this as much as you can.