It's difficult to say an average time, because the federal states are in charge of enforcing the residence law, and therefore it's always difficult for us to provide figures. This is not a category we actually look at. It differs very much according to whether or not the person is documented.
It's easiest when we find these people at the airports, and then they can be sent directly back to the country of departure. But of course if someone is undocumented, it can take months or even years to return this person, and sometimes it's not possible at all if we do not manage to actually identify the nationality of the person.