The psychologist Hermann Ebbinghaus discovered in 1885 that people forget 80% of the newly learned material with 24 hours. His research of the capabilities of human memory led to the so-called forgetting curve. This curve describes the percentage of learned knowledge people can still reproduce, plotted against time. He developed a recurrence scheme with which you are able to store information in the long-term memory.
An important principle that contributes to this is the “spacing effect”. If you want to store information in the long-term memory, you must repeat this with certain intervals. The spacing effect describes how long the intervals should be to to make it as effective as possible. Repeating information to soon only affects short-term memory and can even be counterproductive for the long-term memory.
For good results, the intervals between repetitions need to be just long enough to increase as time progresses. The knowledge is now stored in the reference memory and the chance of forgetting this knowledge again is getting smaller.