I think it'll depend on what you class as usable. I'm happy to run my tyres until they get down to the wear bars, even though they're very squared off (to the point where they can almost stand up straight when off the bike) but I was mainly doing motorway miles so a squared off rear tyre wasn't an issue.
If I was to be more interested in good handling, and riding more twisties, I'd be replacing them much sooner - probably <10,000 miles (my last tyres went to 19,000 miles)
I think this is why some get much higher mileages, but also why some notice a massive difference in handling when they eventually replace tyres - my last tyres change initially felt like the bike wanted to fall to the side compared to the old ones.