For example, you could spend 5 hours in a day revising, but not really covering a lot of content. On the other hand, you could spend 3 hours another day and do the same amount of work.
I would say you should aim for 4 hours of revision per day, taking regular breaks and then more if you feel you're becoming distracted. There's no point ploughing through if you're not focused or motivated, as you'll only drag out the amount of time you'll spend doing one task.
Good luck for your A Levels though