I had a week to revise 8 big topics/political theories for my A-level politics exam. I left it late because I was also revising for my English Literature A-level and AS-level retake, so I was re-reading all the texts which was so time consuming! And I was so engrossed in Game of Thrones. I did not sleep more than 2 hours a day during that week, the night before the exam I did not sleep at all. The exam questions were like a gift from God, all the answers to the questions were fresh in my mind. I wrote down all my Mnemonics and flipped through my notes in my mind that I had memorised and supplemented my essays. Walked out with an A overall.
I would never, ever do it again though. I don't like cramming, I remember nothing afterwards. I never cram now at university.
EDIT: By revise I mean learn the content since I did not attend more than 4 lessons nearing the end of my A-levels.