I did mine a year ago.
I revised about 2 weeks before, spending an uneven amount of time on certain subjects depending on what I liked. I did night before revision in Physics (having not done any work at all that year) and scraped the A because I had 28/30 on my coursework. I revised the night before and morning of my Biology exam, though I possibly deserved this grade more than my Physics because I had worked hard throughout both years on classwork and homework. For Eng Lit I did 3 days before, and was really pleased with my A* because I felt I had put the effort in. Lang/R.E/Maths/French you can't really revise for and I got A*A*AA respectively.
For Geo, Chem and Hist I did more revision, but I think if you work consistently throughout the year you don't really have to revise that much, aslong as your coursework is decent.
And tbh I'm not lying when I say I didn't revise as much as I should have. But I did generally 'underperform' as I got 5A*s and 5As in my mocks, and achieved 6a*s and 4As in my actual exams, slipping off the A* in Bio too. I'll admit I did MUCH more for AS. I'm just lucky that I have a good memory. For GCSE the specification is your bible!
AS and A2s on the other hand are much harder and you do need to do practise papers before.