Start now, if you run out, use other exam boards (especially for maths). I'm doing Edexcel but I decided to do an OCR paper and they were the same types of questions. The only difference might be the mark scheme, how forgiving they are with marks, etc. the specs are very very similar. If you want to see the differences go to TLmaths' website and find his google spreadsheet. You can select what exam board you are taking. I did this and the total number of videos decreased by like 10 (out of about 1000) so there really isn't much change.
Also, you can just redo exam papers you have already done, it helps with time management, exam technique and helps identify your weak points, if you redo them a week or two after you originally did them, you probably would've forgotten the questions by then.
Good luck and start asap!!!
Also one of the best resources I've ever used is TLmaths - watch his videos!