I sat my GCSEs in Summer 2016 and, oh goodness this sounds like bragging, I really feel that I got the revision right for me (my results were a tinsy bit disappointing but that was due to extenuating circumstances).
We had mocks just before Christmas which I revised fairly hard for which were a great stepping stone to see how much I knew/understood already. I then had a break over Christmas (apart from completing any homework) which I think is really, really important, especially if you've gone all out for mocks. I then started light revision (and I mean light, reading over anything I struggled with etc) in January and cracked down to proper revision I think in February half term and then went all out, revision crazy from mid-April.
It's worth mentioning that I'm the sort of person who NEEDS to revise and also hates being unorganised so it worked much better for me to start early and slowly build it up. I had a friend who went all out crazy mad from January and was a highly unpleasant person for 6 months, Her results WERE amazing but she lost pretty much all of her friends and, in the end, says it wasn't worth it.
Basically, do what feels best for you but don't leave it to the last minute OR work too hard. Breaks are important and your health will always be more important than grades!