I personally wouldn't bother! The absolute earliest anyone started revising that I know of was late January, then a lot more started casually revising in February. If you feel confident, start properly revising around March/April at the latest. This summer will be so chilled compared to the ones ahead! Don't even think about it until after the December holidays. If you have mocks in December, don't revise too much but definitely go over your notes a bit; those mocks can be seen as an indication of what you will achieve with minimal effort (in my eyes anyway!)