I also say it takes as long as it takes. At my school, it depends on a person's previous martial arts experience, experience with other physical activities (sense of their body in motion), frequency of attendance and commitment to studying seriously outside of class. Some people have moved to shodan 4 years or so; it took me nearly six. It's not a denotation that means much, in the end, except to show that you have been studying seriously for a while.