There is no fixed number of hours that guarantees retention. What works is focused blocks of about 25 to 50 minutes with short breaks, testing yourself instead of rereading, and coming back to the same material on later days. In Roediger and Karpicke (2006), students who recalled a passage instead of rereading it remembered 61% a week later, against 40%.
| Question | Practical answer |
|---|---|
| How long per session? | 25 to 50 minutes of focus, then a 5 to 10 minute break |
| How many hours a day? | As many focused blocks as you can do well; quality drops before hours run out |
| What to do in each block? | Practice questions, flashcards or writing from memory, not rereading |
| When to review? | About 1, 3 and 7 days later, then weeks apart (spaced repetition intervals) |
Block lengths are common practice rather than a lab finding. The research on testing and spacing is collected in our information retention statistics.
Rereading a textbook, highlighting and glancing over notes feel productive because the material looks familiar, but they build little lasting memory. According to The Learning Center at UNC, "Re-reading leads to quick forgetting."
The problem is recognition. When you reread a page, you recognize the words and feel that you know the content. However, recognition is not the same as recall. Recognition is the ability to identify information when it is presented to you, whereas recall is the ability to retrieve that information from scratch without any cues. That is how students can feel confident while studying and still blank in the exam.
So spend your time on active recall techniques: getting the information out of your memory, not looking at the page again. For reading in particular, see how to retain information while reading.
Active recall means pulling information out of your memory, for example by answering a question or writing what you remember with the book closed.
The "testing effect" is the finding that retrieving information makes a longer-lasting memory than studying it again. In Roediger and Karpicke (2006), students who read a passage once and then recalled it three times remembered 61% of it a week later, compared with 40% for students who read it four times.
Consider the difference between a multiple choice question and an essay question. In a multiple choice scenario, you only need "recognition" (picking the right answer from a list). In an essay, you need "recall" (generating the answer from nothing). Recall is harder, and it is the kind of practice the testing effect is about.
To maximize this effect, you should use a 3-step active recall method that involves closing your book and writing everything you remember before checking the source.
If you spend two hours highlighting a chapter, you have spent 120 minutes in "recognition mode." If you spend 30 minutes creating flashcards and another 30 minutes testing yourself on them, you have spent an hour in "recall mode." The research on the testing effect favors the hour of recall, even though it is half the time.
If active recall is the "how" of retention, spaced repetition is the "when." Hermann Ebbinghaus measured his own forgetting of nonsense syllables in the 1880s. An hour after learning a list, he needed about half the original work to relearn it. A day later he saved only about a third of the work, and a month later about a fifth (Memory, chapter 7). Forgetting is fastest right after learning.
So review soon after learning, then leave longer gaps each time. Five one-hour sessions on five different days beat five hours in one day.
Tracking these dates by hand gets hard with a lot of material. Flashcard apps like Anki schedule each card for you, based on how well you remembered it last time. See this AI-powered workflow for one way to set it up.
For more, see new spaced repetition trends and how Anki FSRS schedules reviews.
Cognitive load theory says working memory can only handle a little at a time. When material arrives faster than you can process it, less of it sticks.
There are two primary types of load to consider. Intrinsic load is the inherent difficulty of the material (e.g., organic chemistry is naturally harder than basic vocabulary). Extraneous load is the unnecessary mental effort caused by poor presentation, distractions, or inefficient study tools.
In a 2023 study at Harvard Medical School (Fischer et al., BMC Medical Education), first-year students at first did not spend the most time on the hardest material. After the course team redesigned the prep materials using cognitive load principles, students moved their study time toward the difficult content, with no rise in total workload. Clear materials leave more effort for the hard parts.
Stop for a break when you keep reading the same sentence without taking it in. More time in that state adds little. Take a break, or use AI tools for notes to break complex information into smaller, more manageable chunks that fit within your working memory limits.
For one topic with an exam two weeks away, spread the work out and spend it on recall:
Very long sittings, like the viral "candle study" trend of studying until a candle burns out, work against you. Focus does not hold indefinitely (see how long students can actually focus). Use breaks instead, for example the Pomodoro method (25 minutes of work, 5 minutes of rest) to keep your mind fresh.
Educational neuroscientist Tracey Tokuhama-Espinosa, interviewed by Harvard Summer School, says that connecting material to yourself makes you more likely to recall it, and that "your brain would love to learn through all of your senses." So instead of one marathon session, try explaining a concept to a friend or using different media (audio, visual, and text) to reinforce the same idea.
If long sessions aren't paying off, check how much of the time is reading and how much is testing yourself, and shift it toward testing.
Writing flashcards by hand takes time, which pushes some students back to rereading. StudyCards AI turns your PDFs and notes into flashcards that you can export to Anki, so more of your time goes into testing yourself.
Try StudyCards AI FreeThere is no fixed number of hours. What matters more is how you study: short focused blocks with breaks, testing yourself instead of rereading, and reviewing the same material again on later days. A few spaced review sessions beat one long session of the same total length.
There is no hard cut-off, but focus drops during long unbroken sessions. Many students work in blocks of about 25 to 50 minutes with a short break between them. Stop and rest when you keep rereading the same sentence without taking it in.
It usually takes several reviews spread over days and weeks. Each successful recall makes the memory last longer before the next review is needed, which is why schedules such as 1, 3, 7, 14 and 30 days work.
Testing yourself. In Roediger and Karpicke (2006), students who read a passage once and then recalled it three times remembered 61% of it a week later, compared with 40% for students who read it four times.
Start early enough to review each topic at least three or four times on different days before the exam. If you only have a few days, spend most of the time on practice questions and flashcards rather than rereading notes.
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