Twice is the most common number of times in most settings. The first test should be scheduled to provide diagnostic information (admissions, hiring, evaluation) and the second should be timed to examine gains as a result of your educational program. Testing can be done annually if these data are needed. As with all testing, care should be taken to motivate test takers to provide a true effort.
Motivating test takers for a critical thinking assessment
Pre and Post testing: Measuring Changes in thinking skills