I tutor GCSE Maths but it would be hard to say how much work you need to do without knowing how much you know already. If you are starting form scratch I would try to fit in several hours a week for several months (itf you have it) - depending on what tier you are doing the higher tier has alot more content and will therefore need more time spent on it. I would recommend doing some past papers to judge for yourself how much work you need to do and then make a plan. Whatever you do I would recommend doing it regularly rather than trying to cram it at the end as it will go in and stew around each week when you're not working and when you come back to something it will seem less tricky next time round. Good luck.