Its hard to say how many miles you can get because I've noticed different tires having different lifespans.

The rear tires do seem to wear out faster

Sometimes I have a run of flats due to a sliver of metal or glass that is in the tire rubber. Sometimes you can run your thumb over it and not feel it because it is small and deep in the tire. Then you air it up and ride and it can give you a slow leak. That kind of stuff is really hard to find and sometimes just replacing the tire is the quickest solution.

I replace rear tires when a hole appears in the rubber and I can see threads. That's basically a tire that has seen its day. I keep those tire around though for winter time and put them on my wheel I use for the trainer so I don't have to chew up good tires. Plus, if you flat in your living room, you only have to walk as far as the kitchen to grab a beer, so its a low risk thing.

I also keep tires around that aren't completely useless in case I have a tire completely go to hell and the LBS is closed for the rest of the weekend.

That's at least how I justify keeping a pile of old tires around