Do what works for you.
Maybe start off with 2 hours a day, as you plan to do at least 15-30 hours in total for the GSCE subjects, and then gradually increase hours as you get nearer and nearer, perhaps 3 hours - 4 hours.. (Around March - April). My personal advice is that the amount you do doesn't matter. It's how efficient you use your time that does.. You could do 300 hours in total for GSCE's exams but have less good results than someone who did 50 hours overall for GSCE's. Work smarter, not harder.