Really depends person to person honestly, personally I - and I know this isn't gonna help - crammed last minute using CGP revision guides and managed to get a 7 in Chemistry and Biology and 8 in Physics. I'd say what matters most is paying attention in your lessons, covering what you really don't understand using revision guides and actually asking your teachers and understanding how your answers are meant to be set out. God knows how long it took me to get it through my head to write out the 6 markers in bullet points rather than one big paragraph, true lifesaver.