If they build him up to be a credible Champion there's no reason he can't hold it for 6+ months in my opinion, it's only the Smackdown belt after all. As much as I want to see Wade Barrett vs. Sheamus in a World Title feud, they've given Dolph a shot with the case, they may as well let him attempt to run with it like they did Bryan.
With Sheamus in a dominant place in WWE as well as Randy Orton, Alberto Del Rio, Big Show and potentially Wade Barrett and others in the WHC scene, (most of whom are treated much better then Ziggler)
Dolph Ziggler may be a massive jobber, but he's still won every active Championship in the company with the exception of the WWE Title. That's far more than Wade Barrett can say he's accomplished. The only Title Barrett's ever held was the IC Title, and his reign was one of the most forgettable in history and he was merely a placeholder Champion to give it to Ezekiel Jackson. Ziggler has been treated like Cena compared with Barrett, especially considering Barrett is a lot better than him.