It really differs when you hear from your College, but I warn you that it'll take a while. Your application has to go from your Department, OK'd by the funding body (if you have one), then the Board of Graduate Studies, who blow the dust off it and OK it, and then I think it might go out to your first choice College. Then it might go to your second choice, and then the College pool if needed.
I may have missed/added/reordered the steps but you can see that it'll take a while. Just be patient - if you are a funded PhD and the Department have given the OK then the University have to guarantee you a College place I think.
I think I had my interview in March and found out in July sometime, and that was my first choice College.
Check out the Graduate Union website www.gradunion.ac.uk/forum as they cover all of this all the time...(i think the website is down at the moment though).
Hope that helps.