There is no simple rule for determining how much ventilation you need. For safety, provide cross ventilation by opening a window an inch on each side of a room. It is better to let in some cold air than to take a chance of not having enough air. As an additional safety factor, you should have a firewatch whenever emergency heat is being used. One person should stay awake to watch for fire and to detect the possibility of inadequate ventilation.
Drowsiness is one sign of carbon monoxide poisoning.
If the firewatch feels sleepy, it may be a sign poor ventilation. If you have not already done so, set up some firefighting items near your emergency heating device. Dry powder fire extinguishers will put out most types of burning materials. Sand, salt, baking soda or water can be used on most non-oil materials. A tarpaulin or heavy blanket can be used for smothering flames.