It doesn't make any sense to measure a UX designer's progress at making wireframes.
Wireframes are just a way to communicate with other people and can take many forms, from sketches to mockups to "interactive" wireframes produced with software like Axure. Their purpose is to represent ideas in a form that can be discussed with stakeholders, team members and developers. Do not use them as a specification.
What you should measure instead is the product. If I make amazing wireframes but terrible software, I'm not a very good user experience designer.
Another thing you can measure is process. How does the designer communicate, prepare, and progress toward the final deliverables? If she sucks at communicating, then her wireframes likely won't be very good either. Fix the communication rather than looking at how pretty her wireframes are.
The important thing to understand about wireframes and prototypes is that they're supposed to fail. The whole reason you make them is to get things out of your head and onto paper/the screen for communication and testing. If you expect your UX designers to one-shot these steps in the process, you don't understand the design process very well; without the cyclical process of learning from usability tests and improving based on their results, there's literally no way to refine initial design work. If anything, the number of iterations should be as high as possible because a good designer will look for feedback and revise frequently, especially in an agile setting.
If your UX designers are expected to produce wireframes as deliverables, you're doing something wrong. Step back and discuss with your UX designers to determine what the right deliverables should be and what metrics you should use to evaluate them.