Is impracticality of Short Classes Tested Or It Is Just A Common Belief?
The closest thing I know of to taking a break every 20 minutes that is based on any research is the Pomodoro technique developed by Francesco Cirillo. The purpose of using this technique was to break any task into intervals of normally 25 minutes. A timer is set and after the 25 minutes, you would take a short 3-5 minute break. The idea was that the mind can only focus for so long before it needs at least a brief rest. 

Taking a short 3-4 minute break every 25 minutes during a lecture seems reasonable. However, it requires the teacher to be aware of principles found in cognitive science or to use common sense as they see their students struggle to stay focused and awake.