How cold does it have to be until you can see your breath?

Answer 1

Seeing your breath happens when warm moist air that you exhale is cooled to the "dew point".  The water vapor forms a cloud of water droplets.  The dew point depends not only on the temperature, but on the relative humidity.  If your home is air conditioned in summer, you can take a shower and humidify the air to the point that you will steam as you open the shower door.  This is not because of the low temperature, but because the air is saturated with water vapor.  Since the moist air you exhale is approximately 98.6°F, it is more likely to see your breath at lower temperatures which do not easily hold as much water vapor.

Grandpa

Answer 2

low 40's.