According to the convention, a class in school is basically 40~50 minutes. In social organizations, each class lasts for one to two hours. Class hours, the time unit of continuous teaching, namely 1 class hour, is the time occupied by a class.

The World Health Organization stipulates that a class lasts for 45 minutes, which is determined according to the degree of eye fatigue and the duration of cerebral cortex. The most effective memory duration of human cerebral cortex is 45 minutes. After this time, memory will decline and eyes will be tired. Take a break every 45 minutes 10 minutes, and you can relax for 10 minutes, do outdoor activities, go to the toilet and look at objects 5 meters away.

The time of a class is called a class hour. In China, a class in primary and secondary schools is generally 45 minutes, while that in colleges and universities is 50 minutes. Also known as "class hours".

The unit of measurement of education and training time related to "class hours" is "class hours". Class hours refer to the learning time calculated by class hours. It is generally believed that both non-adult education and full-time university education are 40-minute classes. Therefore, if 30 class hours of training tasks are needed, the effective training time is 30×40= 1200 minutes, 1200/60=20 hours.