On R is My Friend, as a way to procrastinate on his own dissertation, beckmw took a look at dissertation length via the digital archives at the University of Minnesota.
I've selected the top fifty majors with the highest number of dissertations and created boxplots to show relative distributions. Not many differences are observed among the majors, although some exceptions are apparent. Economics, mathematics, and biostatistics had the lowest median page lengths, whereas anthropology, history, and political science had the highest median page lengths. This distinction makes sense given the nature of the disciplines.
I was on the long end of the statistics distribution, around 180 pages. Probably because I had a lot of pictures.
As I was working on my dissertation, people often asked me how many pages I had written and how many pages I had left to write. I never had a good answer, because there's no page limit or required page count. It's just whenever you (and your adviser) feel like there's enough to get a point across. Sometimes that takes 50 pages. Other times it takes 200.
So for those who get that dreaded page-count question, you can wave your finger at this chart and tell people you're somewhere in the distribution.