If he’s ever going to be president, now’s the time. He doesn’t have to look very far for two case studies: Newt Gingrich and Barack Obama.
The former missed his chance to strike while the iron was hot, passing up a race in 1996 when he likely could’ve taken the nomination from the laconic Bob Dole.
The latter went back on his pledge not to run in 2008 because he understood how quickly and ruthlessly media churns through its sensations. You can only be Bieber for a while (although if you have any actual talent or savvy you may be able to reemerge a decade later a la Timberlake).
I don’t think Christie will have the opportunity to re-emerge in four years, though; my sense (as a recent transplant to Jersey) is that he will not be reelected here. All of the adulation of the national conservative intelligentsia doesn’t seem to have affected folks here; indeed, I’ve heard a few call him too big for his britches (not sure if pun was intended).
And so while he could pull a Romney, quit the governorship w/o contesting re-election, and begin running for the 2016 nomination in the last year of his term, I wouldn’t advise it. You can’t catch lightning in a bottle twice.