[ARCHIVE ANSWER]
There is no specific time a K-9 has to arrive at the scene, but the courts have ruled the arrival of a K-9 must not take an "unreasonable" amount of time. I was recently hired as an expert witness, and the motorist walked free after an Evidence Suppression Hearing.
The judge ruled that 45 minutes was an "unreasonable" amount of time for the motorist to wait for the dog.