Hi!
I'm homeschooling my GCSEs and I'm doing 7 subjects (Maths, English Language, Spanish, Chemistry, Biology, History and Geography). I'm also taking them over a course of 2 years. I am doing them through Wolsey Hall Oxford, and they recommend 150 hours of study time over the whole 2 years. I worked it out and have left time of revising and I do about 5-6 hours a day. I would recommend doing about 4-6 hours a day to make sure you're getting the most study time possible to prepare you for your exams.
Good luck and I hope this helps!