We had a few of our hens get frostbite last winter. Just on the tips of their combs and a tiny bit on their wattles. The affected areas gradually shrivelled up. I think that it took a few months. The areas were whitish, then turned black, then shrivelled up and I assumed, fell off.
I believe that the frostbite on their combs was either caused by their getting wet free ranging in the rain one evening just befoe the temps dropped into the single digits, or too much humidity in their coop. We never did figure out which it was, as we immediately created more ventilation in the coop and now I don't let them out during the day when it is raining and extreme cold is forcasted for that evening.
Some on this website have recommended that neosporin be rubbed on the combs and wattles in the winter to prevent frostbite. We used that as a treatment for those that were affected just to be sure that no infection would occur. There was never an open wound though, so I am not sure if they would have become infected.