Time: the earliest you start, the more time you'll have to cover all the topics, from top to bottom. The more familiarized you are with the curriculum, the better, and for that you need time. Official statistics say that you need on average 300 hours for level I, I needed 600 hours approx. If you take it in June, some people start studying in February. For me that's not enough. I think that 6 months is ok.