Right hang on a minute.
It depends on the person and it depends on the subject.
But basically (and I'm saying this as a teacher btw), you can still make a few mistakes in exams and come out with a 7. The kinds of mistakes you make when you end up with a 5 are much more basic issues, which are often easier problems to solve.
Trying to move somebody from a 7-> 8-> 9 is trying to amend and fix what is already a very good style of working and improve it even more. Much smaller mistakes drop you from a 9 to an 8.
It's all achievable, but there's no particular way to measure what "enough work" is to get to a 9. It's not like if you put 5 more hours a week into revising it must lead to an improved grade. It might, and it probably will, but there's not a set time length required in order to achieve high grades.
Some people are lucky and sleepwalk to 7s and 8s on their general intelligence. Others (like me) have to work very hard to reach that.