I am also year 13 student and doing AQA biology, chemistry and maths. I think that earlier we start better it will be so that we can start as many as we want.
and It depends on ourselves that how we manage time.Personally, I feel that there is not enough time so we have to start soon then we can revise again and again and feel more confident.
And doing past questions is not as much effective as papers I believe.I only do past questions on a particular topic when I am not confident with that topic but doing past papers is much better as in every next question they are asking about something new so you can point out your weakness....
That's only my experience.......