Bobcat pretty much has the right idea. You do have to gradually stress it, and you will get some pain from the periosteum remodeling, but this shouldn't be a nagging thing. In my experience it might seem a little bit irritated (and i stress a LITTLE) but it was fine the next day.
The MAJOR concern with running on a femoral stress fracture is that if its high on the femur (neck or higher) you are at a large risk for developing avascular necrosis. There normally has to be some sort of displacement of the femur in order for this to happen, but it is entirely possible to do this if you're not properly resting it.
I guess what it comes down to is this: Is the fracture healed? If so you want to really gradually increase your mileage. I got off the crutches on january 20 or so, and i'm just getting back up to 50 miles per week now. Its not worth it to rush back. I'm assuming you're young and would like to run for you entire life.
Hope this helps a little.
Ed