I studied for my GCSE for about 4 or 5 months before the exam, as it took time to arrange it through the school seeing as it was an extra GCSE and not usually offered in school. But even so, I achieved an A.
I used the “Astronomy for GCSE” (2006) by Patrick Moore and Chris Lintott, which (with some external help) was sufficient for the learning and, later, the revision.
It is an excellent GCSE to take if you are interested in the subject, and as long as you have a teacher who is willing to provide some assistance with the work and then to observe and mark the coursework, it is well worth the hassle. One recommendation is to try and start it in plenty of time, so either along with your other GCSE in Year 10, or as soon as possible in Year 11.