I'm guessing the interviews depend on subject, but my health psychology interview was about 10 mins long. It was the usual questions - why do you want to study this course, what was your third year project for undergrad about, what kind of project would you like to do, what do you hope to do after this course. They also asked a pretty nasty stats question (especially nasty seeing as this was totally on the spot, and stats exams tend to be open book anyway so there isn't any real need to learn them off by heart)
They did also ask about how I was going to fund the course, I'd imagine they'll be asking more people this as the course fees have just gone up?
But it was pretty informal, and as long as you have researched into your course and what you want to do with it, its fine.