if you are a quick learner then 3 days is the max you need per exam, 1.5-2 days to learn the content and the rest is for past papers, and for some exams you dont even need to do revise (e.g. English as the skills you need are usually learnt throughout the year). The key is to at least learn the exam technique (past paper answers etc....) and then to learn the content around it (since many questions come over and over again in different forms, this is particularly true for maths and sciences), this only works for GCSE btw not for a levels. So I would suggest starting around 2 months before exams, atlhough you could start now and compress all your notes into something which you could read over a day or two before the exam. Good Luck!