Spaced Repetition Schedule: How Intervals Work and When to Review

"Space your reviews out" is solid advice — until you actually sit down to study and realise nobody told you when.

Tomorrow? In three days? Next week? And when you inevitably miss a day, does the whole thing fall apart?

The answer comes down to intervals — the gaps between your review sessions. Get them right and a 15-minute daily habit can lock knowledge in for months. Get them wrong and you either waste time reviewing things you already know or forget things you studied last week.

Here is how intervals actually work, why they expand, and how to build a schedule you can stick with.

Why Intervals Matter More Than Total Study Time

Most students think about studying in terms of hours: "I studied for three hours last night." But memory research consistently shows that when you study matters more than how long you study.

Consider two students who each spend 60 minutes reviewing the same 50 flashcards:

• Student A reviews all 50 cards three times in a single hour-long session.
• Student B reviews all 50 cards once today (20 minutes), once in three days (20 minutes), and once in ten days (20 minutes).

Same total time. Same cards. But Student B will remember roughly twice as much on a test four weeks later. This is the spacing effect — one of the most replicated findings in cognitive science. The gap between sessions forces your brain to reconstruct the memory each time, which strengthens it far more than repetition within a single session.

The interval is not dead time. It is when the actual learning happens.

How Expanding Intervals Work

The core mechanic of every spaced repetition system is expanding intervals: after each successful review, the gap before your next review gets longer.

The logic is straightforward:

1. You learn something new. You will forget it fast — within hours if you do nothing.
2. You review it 1 day later. The act of recalling it strengthens the memory. Now it will last a few days before fading.
3. You review it again after 3 to 6 days. The memory is stronger now. It will hold for a week or two.
4. You review after 2 weeks. Then a month. Then two months. Each time, the memory becomes more durable and the required interval grows.

This expansion tracks how your brain physically stores information. Each successful retrieval strengthens the synaptic connections behind the memory, increasing what researchers call memory stability. A more stable memory decays more slowly, so it can survive a longer gap before the next review.

The forgetting curve maps exactly how fast memories fade. Expanding intervals exploit that curve by catching each memory right before it drops below a usable threshold, then boosting it back up. Over multiple cycles, the decay rate slows and the gaps can safely grow.

Three Approaches to Scheduling Intervals

There is more than one way to schedule reviews. Here are three approaches, ranked from simplest to most effective.

1. Fixed Schedule (Manual)

The simplest approach: pick a set of intervals and apply them to every card.

A common fixed schedule:

• Day 1 — first review
• Day 3 — second review
• Day 7 — third review
• Day 14 — fourth review
• Day 30 — fifth review
• Day 60 — sixth review

Pros: Easy to understand and implement with nothing more than a calendar. Cons: Treats every card identically. An easy vocabulary word and a complex biochemistry pathway get the same intervals, which wastes time on easy cards and under-reviews hard ones.

This approach works in a pinch. It is far better than cramming, but it leaves significant efficiency on the table.

2. The Leitner System (Box-Based)

The Leitner system adds a layer of adaptivity using physical boxes. Cards you get right move to a higher box with a longer interval. Cards you get wrong drop back to Box 1.

A typical Leitner setup:

• Box 1 — review every day
• Box 2 — review every 3 days
• Box 3 — review every week
• Box 4 — review every 2 weeks
• Box 5 — review every month

This is smarter than a fixed schedule because difficult cards stay in the early boxes and get reviewed more often, while easy cards graduate to longer intervals. It is a solid system for paper flashcards.

3. Algorithm-Based (SM-2 and Descendants)

Modern flashcard apps use algorithms that calculate a unique interval for every single card based on your history with it. The most widely used is SM-2, developed by Piotr Wozniak in 1987 and used by Sticky, Anki, and many other apps.

SM-2 tracks three values per card:

• Ease factor — how natural the card feels to you (starts at 2.5)
• Repetition count — how many times you have recalled it successfully in a row
• Current interval — the number of days until the next review

After each review, the algorithm multiplies your current interval by the ease factor to calculate the next one. If you rate a card "Easy," the ease factor rises and intervals grow faster. If you rate "Hard," it drops and the card gets shorter intervals.

The result is that two cards in the same deck can have wildly different schedules. A card you find intuitive might be scheduled for 90 days out, while a card you keep confusing appears every 3 days. See how Sticky implements this.

Building a Practical Review Schedule

Enough theory. What does a real schedule look like week by week? Here is a concrete plan for someone adding 10 new cards per day.

Week 1: The Ramp

• Add 10 new cards per day
• Review yesterday's cards (10 cards on Day 2, 20 on Day 3, etc.)
• Daily review time: 5 to 15 minutes, growing as cards accumulate
• Focus: getting the habit right, not volume

Week 2: First Wave Returns

• Cards from Day 1 start coming back for their second review (3 to 6 day intervals)
• Daily review count: 60 to 80 cards (new cards plus returning cards)
• Daily review time: 12 to 18 minutes
• Focus: rating cards honestly — do not tap "Easy" on cards you hesitated on

Weeks 3 to 4: Steady State

• Review count stabilises around 80 to 100 cards per day (the 8 to 10x multiplier in action)
• Daily review time: 15 to 20 minutes
• Some cards are already on 2-week intervals; others are still showing up every few days
• Focus: consistency over perfection. Six days per week beats seven days for three weeks then quitting

Month 2 and Beyond: Compound Growth

• Your earliest cards are now on month-long intervals
• New cards continue at your sustainable rate
• Total known cards: 200+ with high retention
• Daily review time: still 15 to 20 minutes (mature cards take less time per review)

The key insight: your daily time stays roughly constant even as your total knowledge grows. Mature cards need infrequent reviews, so they barely add to your daily load. This is why spaced repetition scales where cramming does not.

What Affects Your Optimal Intervals

Your optimal intervals will not match someone else's. Here is what shifts them:

Material Difficulty

Simple paired associations (word → translation) tolerate longer intervals than complex conceptual cards. If your cards test multi-step reasoning, expect shorter intervals and slower graduation to long-term.

Your Prior Knowledge

If you already know biology well and are adding advanced anatomy cards, your intervals will be longer because existing knowledge provides scaffolding. A beginner in the same subject will need shorter intervals for the same material.

Sleep and Stress

Sleep is when your brain consolidates memories. Poor sleep or chronic stress shortens the effective life of each review, meaning you need shorter intervals to maintain the same retention. During exam season, this is a real factor, so plan accordingly.

Rating Accuracy

The algorithm only works if your self-ratings are honest. Tapping "Easy" on a card you actually hesitated on gives the algorithm bad data, leading to intervals that are too long and surprise failures weeks later. When in doubt, rate conservatively.

Active Recall Quality

Genuinely retrieving the answer from memory before flipping the card produces a stronger memory trace than passively recognising it. If you find yourself glancing at the answer before committing to your response, the interval the algorithm assigns will be longer than your actual retention warrants.

Common Scheduling Mistakes

Starting with Too Many New Cards

Adding 30 new cards on Day 1 feels great. By Day 10, you owe 300+ reviews and the whole thing collapses. Start with 10 or fewer and ramp up gradually.

Ignoring the Backlog

If you miss a few days and come back to 200 overdue cards, do not add new cards until the backlog is cleared. New cards only make the pile worse. Prioritise overdue reviews — they represent knowledge that is actively decaying.

Reviewing at Random Times

Consistency matters. Reviewing at roughly the same time each day builds the habit and gives the algorithm predictable data. Morning reviews work well for most people because you are alert and the reviews are done before the day gets busy.

Overriding the Algorithm

If the app says a card is not due for 18 days, trust it. Reviewing cards early gives minimal benefit and crowds out cards that actually need attention. The algorithm has seen your entire history with the card — your intuition has not.

How to Start Today

You do not need to understand the maths. Here is the fastest way to start:

1. Pick your tool. An app like Sticky handles all interval calculations automatically. If you prefer paper, use the Leitner system with five boxes.
2. Create 10 cards. Focus on the material that matters most: upcoming exams, core vocabulary, key concepts. Follow the principles in our guide on creating effective flashcards.
3. Review tomorrow. Then the next day. Then the next. The algorithm handles the scheduling — your only job is to show up and recall honestly.
4. Trust the gaps. When a card disappears for two weeks, that is the system working. It will come back right when you need it.
5. Adjust your pace. If reviews feel overwhelming, add fewer new cards. If they feel too easy, add more. The right pace is the one you can sustain for months, not the one that feels impressive for a week.

Properly spaced intervals are the most efficient path from "I just learned this" to "I will remember this next year." The schedule does the heavy lifting. You just need to show up.