i f*cked around a lot in year 12 and was getting straight Es for most the year. The week before my AS/ end of year exams in year 12, i banged out several hours of revision a day and a couple hours in the evenings and got all As.
They only really matter for predicted grades, if your school is willing to negotiate predicted grades or if they predict high grades dont worry too much, otherwise, a few hours a day for about a week or two should do the job
good luck