In the United States, PhD dissertations typically range from 100 to 200 pages, with a median length of around 150 pages. In the United Kingdom, dissertations are typically shorter, ranging from 80 to 120 pages.
In the sciences, dissertations tend to be longer than in the humanities and social sciences. This is because scientific dissertations often include extensive experimental data and analysis. In the humanities and social sciences, dissertations may focus more on interpretation and analysis of existing works.
It is important to note that the length of a dissertation is not necessarily an indicator of its quality. Some very good dissertations are short, while some very poor dissertations are long. The most important factor is the quality of the research and writing.
Ultimately, the length of a PhD dissertation should be determined in consultation with the student's advisor and committee.