As well as revising somewhat for your mocks, it'd probably be best to start compiling actual revision notes (posters, flashcards etc.) in January. Then once you feel you have made an adequate amount of notes, start working through past papers for as many subjects as possible. Then, you can start to identify your weaknesses and work on them more thoroughly. Ideally, by the time you've got up to about Easter, you already have lots of notes prepared and a very strong idea of what you need to focus your revision on.
Of course, it's different for many people, and you may feel better starting off earlier or later.
I somehow managed to leave a lot of revision and note-preparation (other than for mocks) up until around Easter, but I still thought I did okay. Although it was more stressful than it should have been.
So that's why I just recommend making notes earlier on, and then focussing on past papers and whatever revision works for you (make sure you figure out how you can best revise by testing out different methods when you go through your mocks).
Just remember that unless you want to get into Oxbridge or whatever, GCSEs aren't the be-all and end-all. Don't stress out over them too much, especially since schools can overstate their importance a little too much, and pressure you at really early stages.
Don't revise too early unless you leave yourself nice gaps for breaks, or you know you can handle it.
The last thing you want is being "burned out" before you do your actual exams.
When you get to the exams, just remember that it's pretty much a past paper, just with a new and unfamiliar date on!
Good luck, you'll be fine!