There are so many factors to consider - the size of dog, age of dog, level of anxiety, anxiety situation (storm, separation, environment, etc.), digestibility of the dog, etc. We have seen our product work on dogs as quick as 15 minutes and last 4 to 5 hours. With others it may take an hour before they see a difference, while lasting only 3 hours. It truly is impossible to provide a concrete answer as there are many variables at play.