Usually it's their choice, where they want to be.
When the highs are below zero, we have gone to keeping the pop hole door closed, though. That's not the low or with wind chill, that's when the high for the day is below zero, like -6 F. At those temps, my chickens choose not to go out. As long as they don't want to go out at all, then we may as well conserve a little more heat and keep the pop hole door from adding a draft at floor level. Plus, then we go into the night with a little more warmth, which is going to be really cold.
If the high is in the single digits or teens they will still go outside, they just won't stay out a long time. As long as they want to go outside even briefly, I let them.
When the highs are below zero, we have gone to keeping the pop hole door closed, though. That's not the low or with wind chill, that's when the high for the day is below zero, like -6 F. At those temps, my chickens choose not to go out. As long as they don't want to go out at all, then we may as well conserve a little more heat and keep the pop hole door from adding a draft at floor level. Plus, then we go into the night with a little more warmth, which is going to be really cold.
If the high is in the single digits or teens they will still go outside, they just won't stay out a long time. As long as they want to go outside even briefly, I let them.