Bioinformatics lies at the nexus of the biological sciences and the computational sciences. Therefore it is sometimes worth comparing these two disciplines.
Marcus Beck at the R is My Friend blog has looked at doctoral dissertation lengths via the digital archives at the University of Minnesota. His data are shown in this box plot. You can search through it for your own favorite discipline (click on the image to make it larger).
He also has several other graphical views in his blog post, including data on masters theses.