How many miles do you get out of your good tires?

We are changing our tires for the second time, first was to get rid of the junk tires it came with. We went from load range e to load range g and got rid of all the constant leaking tires and de-lamination. now they just wore out at about 15k, down to the wear bars. Just want to see if we are average or need to look at other issues, the wear is for the most part even a crossed the entire tire. Our TH is about 18500lbs with 3 axles.