revision honestly depends on the type of person you are.
i have some friends who spend 2 hours a day revising but are barely keeping up with me, but i have a tried and tested secret formula....
get your textbook (for one subject at a time) and start to work through it. no more than 1 A6 card per page (or for every 2 pages for an AQA SCIENCE book).
read the page, read it again and summarise it down to 1 side of A6.
it takes about 6 minutes max per card, and in all honesty you can revise the whole subject in about 4 hours of "revision".
after you are done you will have about 50 cards on that topic. after you have finished all topics all you need to do is go over the cards. just a quick read through is more than enough.