The optimal time to review is when you are just about to forget a piece of information. Everyone learns at a different pace, so it varies from person to person.
We can improve on spaced repetition further by active recall - stimulating our memory for a piece of information.
Increasing the intensity of emotions associated with the memory. Images, sounds, emotions, smells and stories can be strong hooks for memory and recall.
Spaced repetition has been improved further by Sebastian Leitner, a German journalist, in 1972.
Leitner suggested to decrease the frequency of reviews each time you successfully remember information and at the same time to increase the frequency if you didn’t recall successfully.