It's undefined. The power's description is, technically, broken. You'll have to talk to your group and make a common sense judgement call.
What makes the most sense to me is having it end at the end of the Ardent's next turn, representing roughly a full round in which everyone had the option to take advantage of the unnerved enemy. Many conditions imposed by at-wills, and the Ardent's at-wills, last this long. That might not make the most sense for you or your group, however.
Technically, what could the duration be?
So, what do the rules actually say about this having a condition with an undefined duration? There's two valid answers, depending on how you look at it.
Forever. The CA condition has no end condition, so it simply never ends.
Instantaneous, no duration at all. Yes, I mean it. According to the PHB p278 or Rules Compendium p226:
Unless otherwise noted, a power is instantaneous and has no lasting effect. The two types of durations are conditional and sustained.
This power doesn't have a conditional or sustained duration, and in fact there is no duration specified at all. Since the power doesn't specify any lasting effect, it's instantaneous and there is no lasting effect at all - not even combat advantage.
The enemy grants CA at the end of your shove, and that's it - anyone who wants to take advantage of the CA will need to use an immediate interrupt power to do so. It doesn't last, and it ends after the power ends.
The technical side isn't much help here
The technical options are unreasonable and contradict each other, and one is overpowered for an at-will power. Non-technically, you could justify this as lasting until the end of your next turn, the end of the encounter, the end of the day, or forever and ever. Who's to say how long he's unnerved?