What the research actually found
Cepeda and colleagues synthesised 254 studies covering more than fourteen thousand participants and found that distributing practice across sessions beat massing it into one, with the advantage holding across materials and age groups. The finding is unusually robust for educational research: it is not a small effect visible only under laboratory conditions, and it does not depend on the learner doing anything clever. It depends only on the schedule. What makes it practically interesting is that spacing costs nothing — the same hours, arranged differently, produce more retained material. That is a rare thing to be able to say about a study technique.
How long the gaps should be
The follow-up work is more useful than the headline. Cepeda and colleagues in 2008 varied the gap between two study sessions and the delay before the final test, and found that the best gap scales with how long you need to remember the material. Roughly, a gap of about ten to twenty per cent of the target retention interval performed best. Needing to remember something for a week points at a gap of about a day; needing it for a month points at several days; needing it for a year points at weeks. The practical reading is that most students space far too tightly rather than too loosely.
Where it breaks down in real life
Spacing requires knowing when your exam is and starting early enough that gaps exist, which is precisely what a student in trouble does not have. It is a planning technique before it is a study technique. It also feels worse than massing: returning to material after a gap means having forgotten some of it, and that struggle is uncomfortable in a way that rereading a fresh page is not. That discomfort is the mechanism working rather than a sign of failure, but it is the reason people abandon the schedule they made. The Study Schedule Planner builds the gaps for you so the decision is made once rather than every evening.