Unfortunately it is very individual and therefore impossible to answer. I know somebody who is doing 3 tests a day for the last year!!!!!
But I also know sb. who only started preparing kid to the exams 6 weeks before and she passed.
So I am afraid it is up to yours and your childs endurance, willpower, abilities and so on...
I agree with moose. I also know someone whose child didn't start preparing until about 6 weeks before and also passed. My son has a study timetable for the last 12 weeks but he had been doing the odd paper for about a year before that. Now we do an English and VR/Maths VR on alternate nights and all three at the weekend. It seems to suit my son and he's coping quite well with this and his school homework. It really is down to personal preferences/abilities. Best of luck.