As Ollie's already mentioned, it's going to vary based upon your institution and what your committee members want from you. My thesis was relatively short when it was completely done but my previous university & committee were of the belief that it needed to be kept manageable for later (and quicker) publication if we went that route. My current university, however, does not have that viewpoint and students are expected to spell out every single little detail, which usually results in a couple hundred pages.
Another piece of advice: Develop an organization system for your references as soon as you start searching. It will make life easier later in the process.
G'luck!