According to The Student Room, students review 15-20 hours per week for their exams, which can sound a lot until you break it down. You should check for approximately 1 to 2 hours every day in the months leading up to your GCSE. Starting around March 10 and sticking to that calendar will give you plenty of time to prepare for the GCSE. You may decide to take breaks on the weekends, but that means you should increase your review hours during the week.
If you are very good at a GCSE (e.g. mathematics) you won't need to review it for as long as others. One of the main reasons why the amount of revision per day depends on the GCSE is because it may be better or worse in certain areas. Barnaby Lenon, former director of Harrow, the prestigious independent boarding school that educated people like Winston Churchill and Benedict Cumberbatch, recommends that GCSE and A-Level students check around seven hours a day during the Easter break.