It's hard to give a specific temperature that it can handle. This depends on many factors including the layers of clothing, wind chill, type of resting area.
A better estimate is that it can add up to 5-10°F of heat in the bag. This is for a survival situation to keep warm in the event that you don't have any other access to warmth. In other scenarios such as camping, we recommend putting this around your normal sleeping bag to trap the heat in.