I started revising a month ago. How many hours a day?

Doesn't matter how many hours you do. You could spend 8 hours copying notes out of a textbook or 3 hours doing practice papers and learn more in the 3 hours than the 8 hours. It's what you do with the time. You need to use your time effectively and revise well, not just blindly aiming for maximising hours, because then you won't get anywhere.

Physical energy - eating healthily, drinking plenty of water, getting a good solid 8 hours sleep a night, exercising well and taking regular breaks.

Might not work for you, but my schedule is as follows.

On weekdays, I wake up at 5AM, get showered etc, from 6-8 I revise. That's two hours before school. Then when I get home I do two more hours, and then I enjoy some free time. That's 4 hours spaced out.

On weekends, I wake up at 5AM again, I revise from 6-11. Then I revise from 4-7 and enjoy the rest of the evening.

From mid-March, my plan is to use up 3/4 of my weekends towards studying.