Many of my former students come back years later and ask my advice about getting a PhD. I generally tell them that a PhD program is like a monastery – you have to love the pursuit of knowledge, for its own sake, to make it bearable. If you are doing it only in pursuit of a post-graduation goal, it is too hard a life.
This article includes a startling graph on time-to-graduation. I graduated from MIT in 1982, after 4 years. According to the graph, the average time in social sciences then was 8 years?! I had a lot of breaks (NSF Fellowship, stipend from one of my thesis advisors, pregnant wife to provide emotional support and incentive!) but 4 to 5 years seemed like the norm in my program.
In any case, the second half of the article has some realistic advice about the stresses of protracted graduate programs, and about the importance of your particular advisor’s style.