I applied through college or university. The process took 1 day. I interviewed at Citadel (New York, NY) in Feb 2011
Interview
The person before me took long so my 30-min interview started 7 minutes late. I was asked fit questions that I answered really well (got the guy to talk about Citadel Securities' current situation for at least 5 minutes) and then he asked me some accounting questions which I bombed. After asking him questions, I finished 6 minutes early (17 minutes total). Needless to say, I was not called back for the second round.
Applied through University. They came back to me after two weeks. Had two phone interviews, I was rejected a week after the last one. First interview was personal, second was more technical.