I don't really know how one does a part-time PhD. The kind of work that you produce during a PhD requires 4-6 years of full dedication. A part-time masters I could see.
In England a PhD tends to take on the order of 3-4 years if done full time, largely because they generally require a masters as a prerequisite to even start the degree. A part-time PhD tends to take on the order of 5-6 years.
Of course, even doing a part-time PhD, you end up spending a fair bit of time studying (on the order of 4-6 hours/night).
Of course, even doing a part-time PhD, you end up spending a fair bit of time studying (on the order of 4-6 hours/night).