It took me around six weeks before I knew which college I got into. Furthermore you are guaranteed a place at a college in Cambridge. Which college exactly depends on so many factors but usually if by the time it gets to your fifth college and you haven't been accepted yet, the fifth college is obliged to accept you. You will be spending most of your time in the Alison Richard Building anyway as part of your course that college choice makes little difference and if you really want to you can attend formal hall at other colleges if you have friends there etc.
But usually if all the other colleges are full or oversubscribed then usually you will get passed on and end up at one of the postgrad colleges like St Edmund's, Wolfson, Clare Hall, Hughes Hall, Darwin and if you're a girl Lucy Cavendish or Newnham or even Murray Edwards.
Anywhere but Girton I say lol!