You can definitely complete a full PhD program in 2 years or so, with the right circumstances, motivation, a good advisor, and no program hurdles prohibiting it. I finished in 2yrs, 3 months at a state flagship university while holding down a full-time academic appointment, no leave time. The key for me was a high degree of motivation and willingness to stick to a do-able dissertation from the outset. Also, for coursework, transfer, transfer, transfer. For me it solved the problem of waiting on the academic calendar and my schedule. In my experience, many people stumble at the dissertation stage worrying about particular topic tweaks and flavors of study. Choose a topic you care about and come in writing it day one. Don't wait to be told its ok to begin. Come in wheels turning and do not allow administrative delays. It can be done but at time you may feel slightly insane doing so. Being a strong writer will really help, but the right advisor is everything.