The most important thing is to not compare yourself to others! What works for one person may not work for someone else.
I'm sitting my AS Levels (and one A2) this year and I'm planning to start revising over Easter. Don't panic if you haven't started yet!! There's plenty of time I think, but if you're struggling with a certain topic maybe get that out of the way now if you want, but I reckon Easter is a good time to start
Don't leave it too late of course; it's always better to start earlier and take it more slowly than have to rush and stress out if you start too late