My school had an internal exam system at the end of year 12 for our ucas predicted grades, we had to do 2 A2 papers for each subject, I was revising for about 8-11 hours a day for a few weeks, I think it was really bad for my mental health. I ended up being predicted 4 A*s. In my February set of mocks I didn't revise very much and still managed to get A*AAA (it was like the day before for each exam revision lol, some subjects I didn't even do anything) I guess it just depends on how well you retain information and how much you've prepared before hand. I'm still aiming for my 4 A*s so I'm just making resources like mind maps and timelines or doing practice questions for STEM subjects. 8-11 hours of revision arguably worked for me in year 12 but I don't think the mental consequences were worth it.
Good luck everyone xx