It can get down to 0 degrees, usually around 20.
Leghorns can have a problem with frostbite on their combs, I understand, when the temperature gets down into the teens.
But it would make a difference if the chicken is exposed to moisture or is in the wind, etc.
Maybe others in cold areas can better answer your question about how cold it needs to get for a Leghorn to get frostbite on her comb.