
It varies so much from dog to dog, some get better, some don't -but don't give up just yet. Once the respiratory infection has cleared, that alone should help a lot. In my experience it can take a few weeks for heart medication to have a real effect, and the dosage may well need adjusting once or twice as well.
I have had two heart dogs, different breeds and different conditions. First there was Jesper, Golden Retriever, who had Hypertrophic Cardiomyopathy. He showed no signs at all until the day he suddenly collapsed during a walk when he was 8 years old. Like with your dog, he was x-rayed and found to have an enlarged heart. It definitely took weeks and several adjustments of medication with him, but he did get a LOT better. He lived for a further 3 years.
Then there was Rufus, who ironically I bought when Jesper died. He was a Cavalier. As a Cavalier I made sure he had his heart checked every year when he had his annual booster -it is so very common for them to develop heart murmurs after the age of 5. When he was 7, he had no murmur at all. When he was 8, suddenly he had a grade 6 murmur -the worst one possible. That was Mitral Valve Disease. The vet said he would not make it to Christmas that year, which was 3 or 4 months away then. Again he was put onto medication. Rufus then lived a further SEVEN YEARS and died when he was 15 years and 2 months old. He was poorly on and off for the last couple of years, but always happy. We just let him decide what he wanted to do or not. If he wanted to go for a walk, he could, but if he stayed in his bed when the other dogs were going out, we knew he'd rather stay at home and rest. It was his choice all the time, we never forced anything.