Tyre life is always an important factor in running costs, especially given the costs of modern rubber.
I can say for certain that I'm getting just over 12,000 miles out of a front tyre using the standard BT020. This means that I replace them every third service as I'm not fastidious enough to make sure I replace the tyre when it hits the end of its useful life.
Unfortunately, I have no idea how many miles I should be able to get from a rear as I've had four (Yes, four!) punctures. The longest running rear tyre was at 8000 miles when it got a hole in it and, to be honest, it was looking rather worn by then so I'd guess that about 9000 would be about the lot.