I got all 9s as well and I will say effective revision is not about how many hours you spend on a subject. There were some I spent only two hours revising the night before, and I still got a 9. There were others I spent a week revising to get that 9.
You need to identify your weak topics in a subject and allocate more revision time to those topics. For the topics you already understand well, revise them lightly. Also, take breaks while revising. Concentration dwindles with time so a lot of people need a reset every half hour by taking a break for a few minutes. It's all about revising effectively.