Advice from a friend of mine. He heats, exclusively, with a wood stove.
1) Temps near freezing, you need to put at least 10 gallons of water per day into the air.
2) Temps near 10F you need near 15 gallons per day.
3) Temps near or below zero, well, you get the idea.
Using his humidifier, as well as pots of water on the stove, he's been able to keep the humidity between 25%-30% and can keep his temps at or above the 70 mark...