I am hiring another staff person to verify insurance (both new and established patients) as well as work the denials. In order to measure her productivity, approximately how many verifications and/or denials addressed should she be able to complete in an eight-hour day?

You can expect an insurance verification clerk to complete 75 verifications per day, although technology (or lack thereof) can affect this productivity significantly. Technology includes online and automated call systems for checking verification; the more you can do online, the better. I don't know of any denial standards.

My real answer, though, is that you should actually judge her performance on her ability to reduce the number of denials overall and to boost collections. Doing it fast is less important than reducing the total amount of work and improving collections. Accuracy and skill are key, not speed.

For example, better verification means fewer denials means more cash faster. 

Have her focus on tracking what is causing denials, then fixing whatever you can based on those identified problems within your practice.