Most demons come out immediately. We spent four hours one time on ONE demon before IT left. Sometimes the demons will not leave until you find out what their legal grounds are. It takes persistent Spiritual Warfare to cast OUT some demons. You can't fake out a demon, and especially don't want to mess with them if there is any fear in you.
Fear is a demon, by the way.
I was watching the landscaping people at these new apartments. They brought in pallets of grass sod. Then they would pick up a piece at a time and lay it on the bare ground. It would lift right off the stack of grass. Well, some of the pallets of grass sod sat there for a few weeks without being used.