It took me 7 years all together, but I was going very part-time. The last 4 years were spent doing research (lab and field work), and the dissertation was written over the last 2 years of research. In my department, comps weren't a big deal, so I only spent a couple months preparing for them.
I could have done the coursework in less time, but I don't think I could have done my research and the dissertation any faster.