A Witch has to be strong enough to summon a demon. So some Witches will have to work years at it or some Witches are that gifted and don't have to work years at it.
The only way the Demon can leave the Witch is if the Witch is dead. Killed either by the Demon itself or another force. One of the many risks a Witch takes in summoning a Demon that might not like them and it would kill them.