There's no real system in place for it. They just arbitrarily decide. Dana said if Aldo isn't ready by October then Mendes will fight either Edgar or Swanson for the interim belt. Aldo fought in February, that'd be 10 months. Pettis last fought in August last year. And we all remember how long it was before they did anything about Cruz.
SPEAKING OF! Dominick Cruz is scheduled to fight Takeya Mizugaki at UFC178. I am, oddly, actually excited about it. Cruz is a decision machine, but he fights like a weird spaz and it will have been 3 YEARS since he stepped in the cage so I'm ready to see it again. I hope for his sake he's not slowed down by all the injuries, he's only 28 and his entire game was built on speed and weird movement.