I think it was way less than 100 exhales, and the breaths may not have had time to travel away from the pig, especially if she could guarantee there'd be no wind. So maybe she knows how long something can safely be transfigured into a pig or a liquid, but has a simpler rule for the students on their first class.

Or maybe she could create a magical barrier around the pig so it was as safe as if it were in a glass box.