how many hours of revision A level rn?

The old quality over quantity debate applies here. There's no point thinking "I'm going to do x hours of revision". All that means is you'll be looking over at the clock looking to when you'll be free from revising, but that's not how it should be. When planning your revision you should be planning what you'll revise as opposed to how long you'll revise for. Aim to complete sections of work or exercises. Don't just cycle through subjects in hour periods because you'll start cutting corners and feeling rushed if you go over that time. Just take it slow and steady and work through what you need to focus on regardless of time.