WaPo: Volokh: Waiting for the dogs during police traffic stops

WaPo: Volokh: Waiting for the dogs during police traffic stops by Orin Kerr:

Imagine a police officer pulls over a car for a routine traffic violation, such as speeding or driving with a broken taillight. During the stop, the officer develops a hunch that there may be drugs in the car. He contacts a local K-9 unit and requests a trained drug-sniffing dog; when the unit arrives, another officer will walk the dog around the car to see if it alerts to drugs inside. Although the Supreme Court has held that the use of the dog is not a search, the length of a warrantless stop must be reasonable. The officer can’t delay the driver forever.

This raises a question of Fourth Amendment law that has led to a lot of lower court litigation: If the officer has no reasonable suspicion that drugs are in the car — that is, he only has a hunch — how long can the traffic stop be delayed before the dog arrives and checks out the car?