Revision starts in Year 12
A-Level content is cumulative. Year 13 topics in Maths, Physics and Chemistry lean directly on Year 12 foundations, so gaps left in the first year resurface at the worst moment.
The most effective habit is a weekly review: an hour per subject consolidating notes and re-attempting questions you found hard.
A realistic timeline
Treat end-of-Year-12 exams seriously: they often inform predicted grades for university applications.
From January of Year 13, move to topic-based past-paper questions. From Easter, sit full timed papers and review them carefully.
Quality over hours
Active methods win: retrieval practice, spaced repetition and exam questions. Re-reading and highlighting feel comfortable precisely because they are not demanding enough to work.