I think it depends on how much time available you have to study.
In my case, I took the test after ~8 months since I bought the voucher. But I didn't have a constant rythm, some weeks I studied hard and some I didn't even touch the book.
My advice is that you make an effort to study hard everyday for about 2 hours, in a month, using the book and also some mocks. Then I think you are ready to go. Don't start studying and then quiting after, because the work to remember all the things forgotten is not a good thing.