Well, I get 15,000 out of rear tire, and double that on the front. I change tires at 2/32" remaining tread. Most of my riding is solo, and I make good use of the tranny for decelerating, and use both front and rear brakes all the time. There is a reason there are dual disc brakes on the front, because the weight of the motorcycle shifts to the front wheel when slowing or braking. The rear wheel unloads a bit and harder braking will wear the tire quicker.

If tire pressure is monitored monthly at least, and you use engine braking as much as possible, you'll get great tire mileage.

 

You may have to replace the tires. Most places will not mount used tires on different rims for liability reasons.

The amount of miles depends on a lot of things including the type of bike, road conditions, road temperatures, riding habits and tire composition.

I've never gotten more than 6500 miles out of a rear tire - and I've gone through 12 of them in the past two years.