Hi,
I studied History, English Literature and RS at sixth form. I had mock exams quite often so I was revising the content for my exams in December then once again in March. April / May time I prioritised just creating mindmaps from memory and speed-planning exam questions, there is no point in trying to start too early on if you struggle to remember things long-term. I'd say make flashcards and choose quick-review and planning style revision methods, most of the time time is not a big factor but it's how you use it. Quiz yourself on the main topics weekly and hand in plans for essays / go through them with your teachers, it's the revision methods that matter most of the time, not how early or late you start revising.