How do you know?
It is straightforward. Your cardiovascular system becomes capable of more throughput -- your lungs can take in more air (higher VO2max), your heart can pump more blood, etc. What you get is called reserve capacity. People with high reserve capacity can survive illnesses which people with low reserve capacity do not.
That's no theory of why endurance exercise increases reserve capacity in most people. It's jut an observation that this frequently happens. It not even good enough to tell you the optimum duration of your exercise.
I also don't happen to be an believer in the idea that VO2max is the best possible value that one could come up with, when it comes to issues like surviving illnesses.
It took it quite a while to figure out that keeping medical ventilators at a static pressure level is a pretty bad idea. A static pressure level is really straightforward. With better research I think it's likely we find a more complex variable that does a much better job than VO2max.
The question is - am I doing enough exercise?
I intend to provide a worked example for you to work alongside with your own calculations and decide if you should increase or decrease your exercise.
The benefits of physical activity are various and this calculation can be done for one or all of them; some of them include: