Spaced repetition is a technique for remembering information over long time periods. It takes advantage of the empirical observation that the time it takes to forget a fact is proportional to the time that fact is known. This suggests that the best strategy to remember something indefinitely is to review facts with ever-larger (geometrically increasing) gaps between reviews. Following that strategy, a finite number of reviews is needed to remember something for the rest of your life, and that number is small enough that the study time needed to remember a fact indefinitely is about 5 minutes, so long as the studying happens in this spaced-out way.
An important consequence of this is Mike Nielsen’s rule of thumb: if I expect to spend more than ten minutes of my life looking a fact up, I should just memorize it.
I use the Anki app to do spaced reviews. Facts are stored in flash cards, and Anki tracks how long to space reviews on a per-card basis. Cards that I remember have their spacing increased. Cards that I forget are scheduled for earlier review. With a deck of ~2000 cards, I currently spend about 7 minutes per day on reviews.
I don’t use the ‘decks’ feature of Anki: my cards are all in one deck. I tried using decks, but found that when I occasionally fell behind on reviews I ended up catching up on one deck at a time, which meant I was systematically forgetting some topics more than the others.
In addition to facts, I make cards for rules of thumb and life lessons I want to hang on to.