Efficient memorization using the spacing effect: literature review of widespread applicability, tips on use & what it’s good for.

• "if, over your lifetime, you will spend more than 5 minutes looking something up or will lose more than 5 minutes as a result of not knowing something, then it’s worthwhile to memorize it with spaced repetition."
• "don’t use spaced repetition if you need it sooner than 5 days or it’s worth less than 5 minutes"