Start revising now, it may seem early but the time flies by. I personally prefer to start off by only doing a small amount a day, and then as it gets closer to the exams increasing the volume of my revision. I can't really help in terms of the subjects you are doing as I don't do them, though I would imagine that doing past papers is quite important, as it allows you to familiarise yourself with what sort of answers you need to give to questions. At current I am revising an hour an evening for my A2 exams in the summer, (2 hours on weekends), and then during the study periods I have during the day, though I don't get much homework. I would imagine you would get more, so just don't overdo it, if you have to do a lot of homework one night, do less revision that day. Ensure you give yourself a break. Starting now means that you can spread out your revision more, so you can take time off and relax.