I ride mine until I start seeing fibers rather than smooth rubber. If I ever see even a little fiber somewhere and I don't change that tire immediately, I almost invariably regret it. Generally I don't have a problem up to that point though.
Does anyone rotate tires from front to back when you think they are about worn half way? It just seems like the fronts wear evenly (maintaining a curved profile on the wear surface) while the rears tend to wear flat so they middle of the tire wears out prematurely. I always think of it when it is probably too late to rotate and get any good out of it.
I try to but I’m also usually too late to get much good out of it and tires aren’t that expensive (if you don’t want them to be).