There is no exactly specified time. Depending on the circumstances they can wait until they are reasonably certain that the person, or the items, they are looking for are 'on site' before they serve...
The brazen bull history: The Brazen Bull was invented by Perilaus of Athens (a Brass worker) in the 6th Century BC and offered to Phalaris, Tyrant of Agrigentum, as a gift. It was a large brass bull...