Talmud for Proverbs 24:30

He would also say: A person can study Torah for twenty years and forget it all in two years. How so? If he sat for six months and did not review what he had learned, he would begin to say that what was impure was pure, and what was pure was impure. After twelve months of no review, he would mix up the names of the sages. After eighteen months of no review, he would forget the beginnings of the tractates. After twenty-four months of no review, he would forget the beginnings of the chapters. Finally, he would have to sit and be silent. [King] Solomon said about such a person (Proverbs 24:30–31), “I passed by the field of a lazy man, and by the vineyard of a heartless person. It was all overgrown with thorns, its surface was covered with weeds, and the stone wall around it had been destroyed.” And when the wall of a vineyard falls, soon enough the whole vineyard is destroyed.