Echoing others upthread, I did my master's thesis in around 50 pages. In general, math papers tend to be shorter, since five lines of formulae can contain a month of focused thinking. It also sort of depends on what you're including when counting pages -- for computer scientists or engineers, most of the work of the thesis may be part of one or more appendices which you may not want to count as part of the actual body of text.
Generally speaking, if you're worried about your thesis not being long enough, it indicates that you yourself aren't fully convinced by the ideas you're presenting in the text, which is sort of a bad sign. With that said, putting up a hard lower limit on the length of a paper seems like a bad idea to me, but my experience at university has been that most of the staff is very willing to accommodate students who are able to argue that they need some kind of exception from a rule. I imagine an advisor would be open to waiving a page limit if he or she felt that the matter was thoroughly discussed in a student's work.
For the purposes of a paper, it's far more important that the text forms a coherent whole than that it reaches some arbitrary length. I cut about a dozen proofs from my master's thesis -- they were neat, but didn't ultimately contribute to the unified treatment I wanted to present.