We had summer exams in year 10 like in year 7, 8 and 9 so I revised properly for them as I thought it would really help for the mocks in December. For the mocks I started revising early October, but only because I had a lot of stuff on. I would start revising around half term. For the real exams I started revising in the middle of March. If you have a particular subject you really think you need to work on, for me it was history- so much to remember! - then maybe read a couple of pages a night and just try to learn them starting in January... Reading this it looks like I did loads, but I had a lot of breaks. It's a lot less than a lot of people did! I guess it just depends on how long it takes you to learn things, how much determination you have and how long you can revise without getting really bored! Good luck, it's worth it to feel good after an exam!