How Often Should You Review Flashcards? The Spaced Repetition Schedule, Explained

The starting point for all of this is Hermann Ebbinghaus's forgetting curve — a set of self-experiments he ran in the 1880s, memorizing nonsense syllables and testing his own recall over time, which produced the classic pattern of fast early forgetting that gradually levels off. Worth being precise here: Ebbinghaus documented the forgetting curve, but he didn't design spaced repetition himself — SuperMemo's own history of the technique is explicit that Ebbinghaus "never worked over spaced repetition" as a study method. That came later, from other researchers building systems specifically meant to counteract the curve he'd described.

The shared logic across every real spaced-repetition system since: if you review too soon, you're wasting time on something you still remember fine; if you review too late, you've already forgotten it and have to relearn it from scratch. The fix is to expand the gap between reviews each time you successfully recall a card, so each review lands close to the point you'd otherwise have forgotten it.

Paul Pimsleur published one of the earliest concrete schedules in 1967, describing what he called graduated-interval recall: intervals expanding roughly geometrically — 5 seconds, 25 seconds, 2 minutes, 10 minutes, 1 hour, 5 hours, 1 day, 5 days, 25 days, 4 months, then 2 years. It's a real, documented schedule, though built for language-learning recall drills rather than general flashcards specifically.

Sebastian Leitner's system (from his 1972 book) works differently — physical boxes rather than a fixed time table. A card starts in Box 1; get it right and it's promoted to the next box, reviewed less often; get it wrong and it drops back to Box 1. Common teaching versions use three or five boxes with increasing review gaps (daily, every few days, weekly), though it's worth knowing sources vary on the exact box count Leitner's original design used, so don't treat any single number as the official one.

Modern software mostly runs on something closer to Piotr Wozniak's SuperMemo SM-2 algorithm: the first successful review happens after 1 day, the second after 6 days, and every review after that multiplies the previous interval by a per-card "ease factor" (starting at 2.5, adjusted up or down based on how easily you recalled the card, with a floor of 1.3). One schedule you'll see repeated a lot online — "review at 1, 3, 7, 14, and 30 days" — isn't traceable to Pimsleur's paper, Leitner's original system, or the SM-2 formula specifically; it's best treated as a rough, illustrative approximation of the same expanding-interval idea rather than a documented rule from any one source.

The strongest long-term evidence here comes from Bahrick, Bahrick, Bahrick, and Bahrick's 1993 study in Psychological Science, which tracked foreign-language vocabulary retention over nine years. Their key finding: 13 relearning sessions spaced 56 days apart produced retention that matched 26 sessions spaced just 14 days apart — half as many review sessions, spread out wider, held up just as well nine years later. That's real evidence that the size of the gap between reviews matters as much as the total number of times you review something.

StudyPilot's flashcard tool builds on the same underlying idea: cards you get right are shown less often, and cards you struggle with resurface sooner, so the review schedule adjusts per-card automatically rather than making you guess at a fixed interval. Paste in your notes or upload a PDF, Word doc, or slide deck, and it generates a full card set with that spaced-repetition scheduling already built into the review flow — two free sets a week, every card editable afterward, with progress tracking so you can see what's actually sticking.