Mickael,
Totally agree with Dory and Val that 6 months of actual usage is far better prep than reading the docs. That being said, the docs and the videos will help you. You can also read my blog which discusses many issues that you should know about http://www.marketingrockstarguides.com
I've seen people try to take the exam after 8 weeks of study, only to fail. Maybe you can do better, but it's pretty rare. I'd say the absolute minimum is 3 months use with a lot of study.