Look for the wear indicators on the tire. That will give you an idea. For me personally, if I am down to the wear indicators in the middle of summer (does not rain here in summer) I don't sweat it and may use the tires a bit longer. But in the winter rainy months I am more conservative and may even replace the tires a little bit before I hit the wear indicators as to avoid any hydroplaning. In the winter it's just not worth risking an accident and running on the tires when they are worn.