After a quick internet search I found this article. From that article:
While the number of contact hours ranged widely, from five to over 100 hours depending on the study, those initiatives that showed positive effects included 30 or more contact hours. It thus seems clear that effective professional development requires considerable time, and that time must be well organized, carefully structured, purposefully directed, and focused on content or pedagogy or both (Birman et al. 2000; Garet et al. 2001; Guskey 1999).
From my own experience helping lead professional development, I’d guess 30 hours is light. Colleagues & I are currently in the middle of a 120-hour year-long program and the tide is just starting to shift. However, the extent of change expected likely influences how long that change takes & I like to think I have pretty lofty goals.
My hope for 2016: for school districts to stop paying one-time speakers thousands of dollars & for one-time speakers to find a way to put in the time real change requires.