15-20 slides:  Although presentation content may vary, the best practice is to include the following key content in the presentation:

Dissertation title and names of the doctoral candidate, chair, and committee members
Problem studied
Study purpose
Research questions (and hypotheses, if appropriate)
Theoretical or conceptual framework
Most relevant literature
Uncommon terms or phrases
Research method and design
Results by research question (multiple slides, if appropriate)
Implications of study results
Conclusions: the significance of the study
Conclusions: recommendations for future research
Potential study limitations
Reflection on the research process
Plans for disseminating research results