So, if I have to wait on the dog to come to me, and I can hold a driver for a "reasonable period of time", the question becomes; What's reasonable? Luckily, we have the internets to answer such questions and lawyers to ponder those questions. It seems that the esteemed Orin Kerr has tackled the question and provides us with a "sorta" answer.
But as of yesterday, the simple (if simplistic) answer to the question might have unusual mathematical precision, at least if you accept the lower court cases as correct. The Constitution allows the police to extend the stop without suspicion for eight minutes, but not a minute longer.You can go to the link for the full answer, but it seems that one Court has held that eight minutes is reasonable, but nine minutes is unreasonable. That sounds reasonable to me.