>>>The one thing though i would like to know is if it is bad to leave on for extra time?<<<
One can keep black salve on a spot as long as you can handle it. Back in the pioneer days they would leave it on straight on through...they were a hardy lot no doubt.
I will typically start out by applying a pea-sized amount of salve on the questionable site for 72 hours (or longer if it takes that long for it to form an eschar (scab area.))
Once the eschar has formed I will leave it on only for 12 hours during the days. And place a drawing salve on it over night.