When is Jack chief?

Well, Jack breaks off from Ralph, Piggy, Simon, and Samneric, he starts his own 'tribe' that's focused around hunting. Roger calls him chief to begin with, but if you want the moment he broke off, it would be chapter 8, Gift for the Darkness, when Jack says "I'm not going to play any longer. Not with you." (page 140) Jack decides that he's going to be chief of his tribe on page 146.

But I really do recommend reading the book. It's not that bad.