If I remember right my FXSTC's rear tire had little Harley emblems molded into the top of the wear bars.
If you can still see them your OK, for now.
But if you just see the bars are even with the rest of the tire surface, its time to get a new tire.
I don't make a habit of beating the snot out of my bike and tires but they still only lasted about 9500 miles.
If your riding style is average they may last longer.
But even if your really easy on them I wouldn't expect to get too far past the 10,000 miles mark.