The computer model of mastering of the logically connected material is considered, the dependences of knowledge quantity and forgetting time of half of the acquired information from duration of lesson is investigated. The used approach assumes that the educational material is presented as a set of N separate ideas or information blocks. Each block consists of M learning material elements, ordered and connected with logic links. The probability of the given idea reproduction by the pupil is equal to the product of the probabilities for all LME-s making this idea. To take into account reduction of the pupil's working capacity, we admit that coefficient of assimilation is connected with the duration of training by logistic law. The model has allowed to prove the following regularities: 1) the more number of ideas or their length, the less durability of the acquired knowledge and the “semi-forgetting” time; 2) at first the increase of the duration of training cause a growth of knowledge at the end of training and rising of “semi-forgetting” time, and after that these sizes remain practically constant. The quantity and durability of the knowledge, acquired at a lesson, can not surpass critical meaning. It is connected to weariness of the pupil, decreasing of his working capacity and growing of time of the task solving. The program on Pascal and turned out graphs are presented.