Is this for GCSE or AS/A Level?
For GCSE, I started around late April (first exam was May 5th), and did about 4 hours a week. This wasn't due to laziness/etc, but due to a medical condition. I finished with 3 A*s and the rest As. What I'm trying to put forward by saying this, is that everyone works differently. My long-term memory is pretty ****, but I can hold information for short periods (a few days, etc), so I revised close to the exam dates.
If other people are doing 6 hours a day, it doesn't mean you have to.
For AS/A Level? **** knows, I'm so far behind I'm ( _ ) this close to dropping out : ) ) ) )) ) )