every cambridge college is different, but when i applied to Queens the portfolio discussion was pretty informal where you basically talk them through each of the pieces you included. By 'in length' i think they basically mean that you have put some thought into what you have drawn. Did you draw it for a specific purpose? to capture a particular effect or demonstrate a particular point? Maybe it was an experiment with or to explore something you had kicking around your head. Just talk about it, what you were thinking when you did certain things. Doing a quick sketch of your cat on the morning of the interview, for example, would probably be difficult to talk about as you just 'did it'. A final piece from a worked through art project would obviously be a lot easier to talk about. just take pieces that you are comfortable and happy with, it is quality over quantity, though if you turn up with 3 pieces then they're going to wonder why. There is also only a limited amount of time allocated to going through your portfolio so taking everything you've ever done is not that great either, as they simply wont get through it. sketchbooks are good as they show your thinking and progression of ideas, bring your best, but if you think you've moved on from it and its not representative of what you can do then leave it out. there is no formula for a portfolio, in the end its just taking stuff that demonstrate your ability in a few areas, that you are happy with and that you can talk about. Bartlett interviews are about 15 minutes long if i remember correctly from last year, and Cambridge was 45 minutes for me at Queens, but i dont know whether it varies from college to college.
harther - you are not 'totally screwed' but what do you have so far that demonstrates what you can do? They will be looking for drawings as you draw all the time on an architecture course, if its purely 3D then maybe you need to branch out slightly?