Well nice question.
The quick answer is, you will get it when you enter the world of confusion (i.e., you start to doubt on your Profs. books or whatever materials you have/get ... that you used to believe absolutely a while ago just by reading them or hearing them from somebody else...) and start to think independently. That takes 5/6 years and by the time you will be bounded by several responsibilities like wife, kids and so on...

PhD is nothing but the transformation of bones from one graveyard to another, so take it easily.