The best resource I have ever found on the topic is this, which digs really deep in the topic:

- https://www.gwern.net/Spaced%20repetition

That, combined with Frequency Lists (another really cool topic), is what inspired me to create my own learning webapp:

- https://core.cards/

Note: it's for me to learn, so login is a bit broken and you have to try to do it twice for it to work if you want to test it out.