It has to happen by the convention. Bush won't be dropping out anytime soon. He's got a lot of money. He's getting the whole dynasty on the campaign trail now. Even if he won't win, at this point he sees himself as fighting for the party against Trump. He'll stick around long past when he's not viable. He has the resources to play the long game. And that's the only game he can play because he has zero momentum and almost no delegates. He has to pick up enough that, if Rubio and Kasich were to drop out, he can pick up the majority of their support and get into a contested convention.
Granted this isn't likely to happen, as chances are Rubio doesn't fail so hard he becomes Perry. And Rubio is the better establishment candidate. But there's a decent chance Rubio keeps flubbing to the point where he ruins himself. And that's what Jeb! is playing for at this point. A war of attrition with the other establishment followed by a appeal to being the least bad choice among him, Trump, and Cruz come convention time.