I gave my son his first GL practice paper 5 months before the exam and he scored around 50%. My son felt he wasn't good enough for grammar school. I realise this was my mistake as he hadn't fully covered the topics and I was just trying to establish what level he was at. I then went back and covered the areas that he hadn't covered in school, before giving him a second paper in June, in which he scored in the 80s. After around 10 practice papers his marks had increased and were fairly consistent so I thought he'd reached the point of his ability. We covered 16 practice papers in the end, alongside lots of reading, and he very comfortably passed all the tests. Personally, I would have liked to have covered a few more papers with him for peace of mind, but he got to the point where he didn't see the point of doing more.....perhaps the lovely weather and his friends playing football had something to do with it
I fully agree with Oshosh in that all the topics should be covered before attempting the papers and then focus on areas of weakness.