In my day, we did not have GCSEs, we had GCE Os, CSEs and RSAs (1960s). And as I went to a secondary Modern, we were not allowed to do GCE Os.
I did not get very good results, it was equivalent of getting my name right on the paper.
However, once I learned how to learn, I achieved 2 BA Hons and nine professional qualifications.
BTW: it is not about the length of time you study, it is about how you study.
1/ humans cannot concentrate for more than 20mins... take a break every 20mins or everything after that is a waste of time and brain.
2/ remembering is effort after meaning... make the material meaningful to YOU.
3/ use test papers. they can be used to reinforce the work.
The method I teach is: 20/5/5
20 mins study
5mins relax may be even meditate
5mins test on that 20mins study
Do this in three batches, then take a longer brake