"How long does my dissertation have to be?" This question has bedeviled many people who are entering a doctoral program. The answer is not simple, as requirements vary by university and sometimes even within universities. Sometimes there are no length requirements at all. While the format of your dissertation and the intellectual rigors are most important, you'll still want to make sure your paper's length falls within some reasonable limit. Write it too short, and your professors may assume it's incomplete. Write it too long, and they may be bored before they get through reading it.
No universal standards exist for the length of a thesis, since the standards vary within each university system or institution. Some universities do not even specify a required length; if they ask, Ph.D. candidates are advised that the length should be however much is necessary to adequately defend their research. At other institutions, the guidelines may vary within different schools or even departments. For example, the arts and sciences department may require a specified length, but the pharmacology school may not. It is useful to think of your dissertation as your first academic book. Books are usually numbered in the hundreds of pages, not the dozens.
If you approach your dissertation as your first academic book, remember that just as books have a specific format, your dissertation must have the same. You are expected to include a title page, an abstract, a properly paginated table of contents, and a bibliography and notes section. Some dissertations are even required to be indexed. Aspects such as the bibliography, notes, and an index, if required, will substantially add to the overall length of the paper without increasing the text. Some institutions that have length requirements do not include these pages as part of those requirements, while others just take the total page count. Know your university's requirements thoroughly before finishing and handing in your dissertation for approval.
Whatever the length of your dissertation, it's eventually going to face approval by a committee. Dissertations are reviewed by a committee consisting of the dissertation adviser and at least two other members of the faculty. This board is often more interested in intellectual rigor than in page count, but it's still important to follow their length requirements. It might be possible to come up a little short, and still produce an outstanding piece of research that meets approval; however, you would be taking a risk by not following the guidelines.
Hitting the Target
The point of a dissertation is to produce a paper that contributes to academic knowledge in some way. If approved, your dissertation is destined to go onto the university library's shelves. The quality of the research is a key consideration. If there is a minimum requirement of 200 pages, and a dissertation only contains 120 pages, it probably means there are either substantial aspects of the question that have not been examined, or substantial holes in the research. If a dissertation is short by several dozen pages, rather than a couple of dozen, it will appear to be unfinished, meaning that more research is necessary. This is not a problem that will be solved by simply being more wordy and padding your dissertation with fluff. Professors can tell when a dissertation has been padded for length, and your review committee will not reward your mediocre efforts. Spend time in your university's library perusing the published dissertations. Pay attention to how closely the Ph.D. candidates of the past have followed the length guidelines. Follow in their footsteps; after all, they've traveled the same path you're on, and they've done so successfully.
Length & Time
Doctoral programs are structured so that Ph.D. candidates begin their research during their second year, if not before that. The program's coursework is usually over by the end of the third year, even though accomplishing a Ph.D. takes between four and eight years to complete. Even an efficient and hardworking Ph.D. student typically spends two years part time and one year full time working on his dissertation. One of the best ways to reduce the time you'll spend on your dissertation is to choose a topic that has become familiar to you. Typically a Ph.D. candidate will do her dissertation on the same subject matter her coursework is being done in; not doing so will cost you valuable time. Candidates with a master's degrees would do well to use their master's thesis as the basis for their dissertation, sparing themselves as much as a year of part-time work. Any internships, fellowships or jobs taken while working on a dissertation ought to be in fields that will help rather than hinder research. All of these steps will allow a Ph.D. candidate to maximize her research time, producing a paper of the necessary size and quality in the minimum amount of time.