I've heard that radials rot from the inside out in about seven years, so they should be replaced at least that often regardless of the tred that is left.
Bias tires seem to last forever if you keep them out of the sun.
Case in (extreme) point: The front tires on Lurch are over 80 years old. How can I tell? The size is 30x5, which changed to 5-20 in the mid-1930s. See the pic below and shudder.
For safety (?), Lurch has inner tubes and flaps that are only 4 years old. Also, I only drive at around 20 mph. ;-) I've only had one flat with driving these tires for over 25 years.
