It all depends where you are starting from -- I am currently studying so we have the same problem....
If you have good knowledge of UML/OOAD then a week or two should be enough to firm up your skills.
If you are a complete beginner then I expect, with commitment, about 1.5 month should be enough.
Some people have said UML distilled + some experience is enough, but remember the availablity of pre-exams is very low, so it is very hard to judge your readiness.
I have bought a
course which I think should just about cover it, it includes questions etc.
James