To year 11 Watch
Start now! It doesn't mean you have to lock yourself in your bedroom for 5 hours a night. Even something as simple as looking as your syllabus and targeting areas that you need to improve on can help. For example, algebra in maths may be a weakness. You can start identifying where to find resources/books to help you.
Even just casually skimming over your notes and watching a YouTube video can help
Obviously, I don't recommend that, because not many people are good at cramming. Little and often from now will do you wonders, and then the month before your first exam I would recommend starting a proper timetable with a good few hours per day dedicated to work.
If you think you need to do more than that, then that's your decision to make and you should make it based on your mock results.
Take a week's rest after mocks dont stress too much