It takes about 10 minutes in below-freezing temperatures to cause chicken frostbite. The most vulnerable parts for chickens are the combs and wattles writes Mary Christiansen. She also shares ways to prevent damage to those exposed, featherless feet.
Source: Backyard Poultry, November 2, 2021. Link.
Also see: Why chickens’ legs do not get cold in winter, The Omlet Blog. Link. Those skinny, bare legs have scales, which retain heat to a certain extent, but they will still get very cold if the bird stands still for too long.