Yeah I started revising straight away. I started off at the start of year 12 just by re-writing and re-reading my notes each day, and reading ahead before each lesson, then at the weekends I would do one to two hours each day. Then I gradually built up how much I was doing based on when I had class exams or mocks, and then the real exams. It's great to start early because you'll never regret starting too early, but you will regret starting too late! Just don't do too much too soon, or you could burn out and then underwork, it's all about balance, which is different for every individual, so trial and error in the early stages to find whats best for you!