I just read the book, it says "when you use the attack action or full attack action in melee" so you have to be attacking, but as Rene says, it would be in place on the round after you use it, if you didn't take other action after your attacks, and before your initiative.
"The changes to attack rolls and defense value last until your next action."
I wonder then if, you use CE this round with a full-attack action, then someone provokes a AoO which you take, is that AoO considered "your next action" since it happened after your full-attack, and thus ending the CE penalty-bonus?