To be honest, with the increasing absence of AS levels, GCSEs become imcreasingly important as the only set of formal public examinations. Therefore I suggest you treat it as such. I would recommend starting ASAP, or before the month end. I certainly started at some point during March, even if only slow steady revision. I got 11A*s in biology, chemistry, physics, English Lang, English lit, maths (all igcses), as well as French, Latin language, latin literature, geography, history. Message me if you need any specific advice!