That completely depends upon how good you are at maths and what grade you would be aiming for. But it is completely doable if you are indeed good. I taught myself a third A-level in maths and put minimal time into it. For one of the modules, the first time I opened the book was 2 days before the exam, and I still achieved an A* (It was D2 though to be fair! Haha).
So in conclusion, yes very doable, but the amount of effort needed alongside your other studies will depend greatly upon how good you are at maths.