I changed the Kenda rear tire at 16,000 miles and it was not down to the wear bars. SSITS was coming up and I wanted the max traction in the wet so I changed it out. The fronts I changed at 21,000 mile and they were not down to the wear bars either.
My tire was the Stock Slingshot Brand Tire. That is another part of the equation for miles you can get, they brand and quality of tire. FunCycle has brought up a good point, the type and conditions you drive in will be important as to how far down you go with your tire wear. I know FunCycle likes his performance drives, so he wants good rubber on the ground for his many different driving ranges. The more your tire wears the less safety you have for wet road conditions and hydroplaning becomes a bigger factor and the tread groves are not as deep and can not pump out as much water as you hit water puddles. The more aggressive you drive the less wear you will want to place on the tires before you change them out. Keep a look at the wear bars and find the point you are comfortable with.