Gcse maths?

Hi, so I did my gcse last year and I got a grade 5, which allowed me to do what I want in college, but I want to do accounting or something finance/econ related at uni, however at the unis I wanna go to I need a 6/7, back in highschool regrettably I never attended till around 3 months before gcse, anyway what I wanna know is how long it would take me to go from a 5 to an 8? I have around 10 hours free per week to study maths gcse and I’m gonna pay to do an online course + resit. Would it be better for me to resit in November or June? In June I might have more college exams so I might not be able to be as focused on gcse.