The timing really depends on where you're at right now and how you learn. If your grades are okay at the moment, you don't need to panic immediately - but I think "starting late" is honestly the most common mistake I see people make. What makes sense is to start reviewing regularly now instead of pushing everything onto a few months before the exam. I'd say: start now in late summer with maybe 2-3 hours a week, where you specifically work through the gaps from your lessons. It doesn't sound like much, but it's consistent and keeps you from being completely blindsided.
Then ramp it up in autumn to 4-5 hours a week, and as you get closer to the exam, maybe 8-10 hours. That's a rough guideline - some people need more, some need less. The trick that a lot of people miss: find someone who actually understands math (a tutor, a smarter classmate, online resources), and don't just have them explain the problems to you, but walk you through the reasoning behind them. It doesn't help if you memorize how to solve last year's problems - the exam always throws variations at you that you haven't seen before.
One more practical tip: start keeping an error log from now on. When you get a problem wrong or don't understand something, write down *why* you failed - was it a concept, a calculation error, the wrong formula? After a few weeks you'll see your pattern and can work on it specifically instead of just tinkering around everywhere.