It's completely subjective - I had teachers telling me to start revising in February/March and, due to work pressures and other commitments I just never found the time so ended up starting over Easter. For me, this was fine and I got what I wanted - anything longer than 6 weeks and I would have just started forgetting everything all over again :/. I have friends who started a long time before me and did a lot worse, and friends who started after me who achieved the same results I did. It's up to you how you learn and revise best (you must have some idea after your GCSEs) but beware the "superficial" revision - loads of post-it notes and carefully colour-coded spider diagrams that make you feel like you've done loads of revision but, actually, when it comes down to it, you don't know the material. Keep testing yourself!