I guess the simple answer is up to you. Don't let anybody tell you you need to revise every day from now to exams - that isn't possible at GCSE. Similarly, don't revise anywhere near the last minute.
For real GCSE's in June, I probably started somewhere in march. For me it was a case of when everybody started talking about it, and I started a plan of what I wanted to revise, when, and worked backwards until I had a plan for up until exam time. Many people did less intensive revision since February/January/the dawn of time etc.
The mocks are up to you - how confident are you? How much effort are you prepared to put in? For me mocks had less content than GCSE's, so I started probably somewhere in November (but my mocks were in January). Mocks mean very little in the long term, but as you're going for A*'s this extra 'forced' revision time is very useful - especially in learning mistakes and prioritizing areas of revision next time.
I would start now, and build up to more intensive revision near the mocks. If it is too much, tone it down, if you're not getting enough in change your plan to do more. Hours per day depends on other commitments - I think I did too much (5-6 hrs weekdays, 10hrs ish weekends near the end), and ended up a bit stressed. Take a break before beginning proper GCSE revision.
Tom