How many artifacts per mile

In exploring old logging grades, I have gotten pretty good at picking out signs that indicate one way or the other whether the trail I am walking is a RoW or something else. However, there are times when the eye test hasn't been enough to give an absolute answer. I have started using a metal detector to help with my determinations, and though this has helped greatly, it still leaves a few questions. I have used the detector on very short spurs that would have been used for only a short time and found many RR artifacts (spikes, joint straps, bolts and nuts), but I have also used the detector on what I (and others per maps) thought were RoW using the eye test only and come up completely dry of artifacts. So I ask fellow grade explorers, in your experience, how far can a RR grade go with no artifacts buried or laying in the RoW? I am talking about RoWs that are undisturbed, not turned into roads. I know there is not an exact answer, but I would like your opinions. If I explore what I think is RR grade for a half mile and find nothing RR related, should I write it off as being RR?

Brian