You don't become an MVP by taking exams or doing some task, it's more like an honorary title. I'm not entirely sure how you DO become MVP, but you have to be nominated (not sure by who). As far as I know MVP is a reward for contributing to a community dedicated to the subject at hand. So there is no real time frame either, you could become one in a year (unlikely) or it could take several. Or you never be one no matter how hard you try.