The report Board Evaluation - Overview of International Practice by OECD 
answers some key questions regarding board evaluations, including:
What is the role of the board of directors?
What is the most effective mix of director skills?
What are their responsibilities in terms of good governance?
What are the key topics to be addressed in the board evaluation process?
How frequently should a board be evaluated?
Should the evaluation be conducted by a third-party consultant or by an insider? What are the current international practices for board evaluations?
Should board evaluations be disclosed?
What is the role of regulators in promoting board evaluations?

read it here