Why the best gap is not one fixed number
The spacing effect is the well-established finding that memory improves when repetition is spread out rather than crammed together. But the idea of one ideal interval is a simplification. A quantitative model of practice scheduling shows that the optimal interval for each item changes as that item becomes more stable in memory—early on, shorter gaps work better; later, longer gaps are needed. [3] This means the 'right' interval is not a constant but a moving target that depends on how well the learner already knows the material.
A theoretical model of memory reactivation likewise treats the spacing effect as producing an intermediate optimal interval rather than a simple rule like 'longer is always better.' [2] The model reproduces the characteristic peak in retention at some middle gap, but it is a general model of memory, not a prescription for a specific number of days or hours that would apply across all subjects.
The learner's starting point changes the answer
Because the optimal interval grows as items become more stable in memory [3], two learners studying the same material can need different gaps. A beginner who has just encountered a word or grammar rule is in a different memory state from someone reviewing familiar material, so the same schedule will not be optimal for both. This is a direct implication of the scheduling model rather than a tested comparison across populations, but it explains why a single universal interval is unlikely.
In second-language learning, a study of input spacing for grammar found that a longer interval improved retention compared with massed presentation. [4] That result supports the spacing effect in a specific population—language learners—but the study did not test whether the same interval would be best for other learner groups, so it cannot establish a universal number.
What is still unresolved about flexibility
The evidence for flexibility is largely indirect: models predict that the optimal interval shifts with memory stability [3], and a grammar-learning study shows longer gaps help in one population [4]. But none of the studies directly compared optimal intervals across different subjects or learner populations. A study of spacing and relearning for second-language words and collocations focused on the optimal interval within study phases and did not address variability across subjects or populations. [1]
A training-paradigm study noted that prior work failed to pin down a single optimal interval and that hybrid schedules helped some tasks but not all. [5] That is consistent with the idea that the best spacing depends on the task, but it does not settle how much the ideal gap varies between, say, vocabulary and grammar, or between children and adults. The practical answer for now is to treat spacing as adjustable: start with shorter gaps and lengthen them as material becomes familiar, rather than hunting for one universal number.
Sources used in this answer
The Effect of Spacing and Relearning on the Acquisition of L2 Single Words and Collocations from Classroom Instruction
The study examined spacing and relearning for second-language single words and collocations, focusing on the optimal interval within study phases, and did not address spacing flexibility across subjects or learner populations.
The Spacing Effect: A Reactivation Model of Memory Consolidation
A theoretical model reproduces the spacing effect's intermediate optimal interval from a structural requirement, but it is a general memory model rather than a prescription for a specific interval across populations.
Using a model to compute the optimal schedule of practice.
A quantitative scheduling model shows that the optimal practice interval for each item increases as the item becomes more stable in memory.
Input Scheduling and L2 Grammar Learning: Investigating the Delayed Effect of Spacing
In second-language grammar learning, a longer interval between presentations improved retention compared with massed presentation, supporting the spacing effect in that population.
A novel training paradigm for knowledge and skills acquisition: Hybrid schedules lead to better learning for some but not all tasks
A training-paradigm study noted that prior work failed to establish a single optimal interval and that hybrid schedules improved learning for some but not all tasks.