Clueless about the limit, I'd be googling just like you.
I have a few methods of determining if it is time to replace. Number one if I notice the woppity woppity noise when the indicator gets worn to. Number two if I skid out. Simple as that. If I hit a skid then I need better tires because they're not matching my driving condition and style. Since this so very rarely happens, I'm pointing fingers at the tires. So then I go get new tires. Number three is simply when I buy a new used car. I don't trust those tires. I want the Michelin cross-countries. I go get the new tires.
