It must be where I ride, but all tires chunk out on me, and it seems like they all wear out fast if I accelerate hard on pavement before they are hardend, Ive had the best luck with maxxis, that said, they dont have enough sidewall support to get out of first gear if you loose a steer tire trying to get back to the pits, but I think that is what gives me good feedback on how it is grabbing the terain.
as far as when a tire for me is too worn, when the back tire has half tread I pull it off, and when the steer tire doesnt look brand new anymore I pull it off. Im old with a mortgage and I need to have confidence in my tires.