Forgetting curve
As a rough guide, after 20 minutes, people can only recall about 60% of what they memorized. After a week, only about 15% is left. When Ebbinghaus did the experiment, he used meaningless sequences of syllables. In the 20th century, several other researchers repeated the experiment. One of them was Georg Elias Müller. For this reason, the results are still considered to be valid.