When it comes to preparing for GCSEs, The Student Room recommends that students spend between 15-20 hours per week studying. This can seem like a lot, but when broken down into smaller chunks, it's much more manageable. Aim to review for 1-2 hours each day in the months leading up to your exams, starting around March 10th. This will give you plenty of time to get ready for the big day.
You may choose to take a break on the weekends, but that means you'll need to increase your review hours during the week. If you're particularly strong in a certain GCSE subject (e.g. mathematics), you won't need to spend as much time revising as you would with other topics. The amount of time you should spend studying each day also depends on how well you know the material. Barnaby Lenon, former director of Harrow School, suggests that GCSE and A-Level students should review for around seven hours a day during the Easter break.