Agreed. Although I was doing SGs and I'm not sure how they and GCSEs compare, but all I did was brush up on anything I hadn't been paid attention to in class - a lot of my year, me included , spent a lot of time messing around between 3rd year and mid 4th year - and then a couple of past papers for most subjects. Starting two weeks before the exams until the end of the exams (~6 weeks) I estimate I worked every third or fourth day, for a maximum of two hours.. I got distracted frequently.
I thought the exams were fairly easy. Expect 1s but I might get a disappointing 2 in there, knowing my luck.
As long as you have a good grasp on the subject, very little revision is needed, and it is very important not to do too much revision because when you get to A levels, when it really matters, you'll have burned out.
Do as much revision as you feel you need, but don't do anything you'll regret later.