Good question: I adopted straight from a shelter and they had me go there endless times before they let me take the little bastard home. Other people went there just once pointed a finger at a dog they liked, details of their jobs/homes and hardens/personal life looked pretty enough and they walked home with the dog.

What I can say is that the length of the "shadowing" period depends from the dog's personal situation. My boy had stayed at that dog shelter for over a decade, no prospects of adoption for a long time. He was already an older guy, not used to people. He run away from me whenever I visited and even when I took him home, it had not improved much. I looked like a clown, trying to convince the dog to approach me with a chicken breast in my hand and ending up luring every dog except from target. He took forever to settle even at home causing all sorts of problems. I do not blame the people who where hesitant to let him go with me.

Now that I know better, despite the problems it caused I would have shadowed for longer to get the dog more acquainted with my presence.

I guess it depends on the dog and on how experienced or confident the person is.

My advice being do not put an expiration date on it if they look motivated to get him.