how long does a doctoral dissertation have to be

A candidate must present a dissertation demonstrating original and independent investigation and achievement. A dissertation should reflect not only a mastery of research techniques, but also ability to select an important problem for investigation, and to deal with it competently.
For information about submitting a dissertation, please see Thesis/Dissertation.

References:

http://qbc.wa.edu.au/?qtgek=phd-dissertation-help-how-many-pages