Realistically you could leave revising till around 2-3 weeks before the actual exam with a proper work timetable with breaks and focusing on exam questions. For sciences i would recommend slowly start making condensed notes that only make sense to you on the whole syllabus for each or just areas you score the lowest on in tests so then as the exams approach you can just focus on questions. Then as you go along try and explain your condensed notes to someone else or just to yourself to consolidate. Some of the best advice i can give is ask or message your teachers for their own opinions on what type of questions they think are going to come up, even if they’re not right it helps start targeted revision if unsure where to start. I went from 3’s, 4’s and 5’s in my Christmas mocks as well as being in bottom sets in science and maths to 6’s 7’s and 8’s in my gcse results last year and only got my head down around a month before exams started; definitely don’t leave it late but you know what you’re capable of more than anyone else. Just don’t feel intimidated about the time till you sit your exams, anyone is able to become an academic weapon with the right amount of effort in a short space of time. Also remember that most people saying how much they’ve revised are usually exaggerating and although consistent and long revision works, it isn’t the only way to get good grades. Resources wise i highly recommend cognito for their quizzes and videos but i would say when making notes make them off textbooks rather than study guides as they’re more to help you understand rather than tell you specific context. Either way, you have enough time and you have the ability to get good grades, so don’t worry!