Sometimes it depends on the test being performed. My other half works in a medical lab. His department is highly specialized, and sometimes the tests just take longer to result out. It's not always the fault of the test itself. Duke is amazing, but sometimes very specific tests require extra time. For example, if you have samples drawn which are sent to another lab, that's an overnight trip at best. Then depending on how long the test takes to process, add time for that.
Once you're there, the department should be able to give you a time frame estimate for when your results should be available. Don't be afraid to ask. And if the results haven't come back by the expected date, then don't be afraid to call and politely inquire about an update. Just remember that many doctor's offices call with results at times that work for them, which sometimes means lunchtime or after the office has called.
Good luck!!!