A leave is usually anywhere from 3 days to 2 weeks, a furlough is generally longer. In the early days of America's involvement in WWII, my dad was granted an emergency furlough because his dad got pneumonia. Much longer recovery time for that back then, if at all. My dad was the only adult child and had 6 younger siblings at home. With no one else to run the farm, they'd have been in a bad way so the Red Cross arranged a hardship furlough of several months. When my grandfather recovered dad went back to his unit. That furlough kept him out of the North Africa campaign and he never actually saw action till Normandy.