This could be best answered if you take a comparative stance. Try to look at your research results and see how their significance compares with other work in your area. Try to publish, present papers at conferences, and discuss your work in as many spheres as possible to get feedback. Making your work accessible to other researchers and also finding out about other people's work in progress is important. Familiarity with a range of types of studies better allows you to gauge the standard of your own work.

Of course you rely on your supervisor to a considerable extent. One of the roles of your supervisor is to be able to judge and advise on the quality of your work. Your supervisor could be sure your work is good enough for a PhD but thinks this is self evident and just doesn't mention it to you. Perhaps you need to discuss it. Occasionally a supervisor becomes as enmeshed in your work as you do and may not have enough distance from which to judge it objectively. Indeed, the supervisor's enthusiasm sometimes causes him or her to urge you to do more and more - which may be nice but not necessary. Maybe you both need to discuss it with an impartial person or persons.

Whereas some students express this doubt in terms of quantity, in our experience, this question really taps the issue of original contribution, academic standards and scholarship. If these are the focus, then 'enough' in terms of evidence will be addressed. Concerns of outright thesis length, however, do arise and institutional guidelines often give an upper rather than a minimum page limit. For example, the University of Queensland rules state that "the thesis should not be unnecessarily long. Although length may vary according to the topic and the discipline, a thesis is expected to be no more than 100,000 words or 400 pages, including maps and diagrams, but excluding bibliography."