Cardiovascular improvements in terms of endurance and efficiency can start to show in as little as two weeks of consistent cardio training. However, the rate at which you see improvements may vary based on individual factors such as current fitness level, age, and genetics.
For example, someone who is new to running may see faster improvements than a seasoned runner. Similarly, someone in their early twenties may experience faster improvements than someone in their fifties.
Regardless of these factors, consistent and gradual training will lead to an overall improvement in cardiovascular fitness over time (3).