My elderly dog (60 pounds) got kennel cough for the first time from the vet exactly 5 days after going there to get vaccination boosters, including one for kennel cough! I added a few drops of 3% peroxide to his food, mixed with water, and he stopped coughing within about 48 hours. However, I didn't keep up the peroxide and the cough came back.. After a week of this, I took him back to the vet who told me, "Yes, it's possible he caught it here" and that it can take up to 2 weeks for it to run its course, maybe longer for older dogs. He ended up giving me antibiotics since my dog also had a runny nose and I was about to go out of town and leave a coughing dog with my dog sitter. The antibiotics kicked his healing into high gear and he stopped coughing the next day. So I like the peroxide remedy for kennel cough but recommend to definitely keep it up for a few days after symptoms disappear.
I think if one of my dogs gets kennel cough in the future, I'll try the peroxide, honey and fresh lemon juice remedy that I just read about in posts here on EC.