What it sounds like you are asking is how can you improve cardiovascular health most efficiently with the use of activity.
First off, heart health is mainly achieved through dietary adjustments but regular activity plays a vital role as well.
Any activity can improve the health of your heart provided you are reaching the right intensity. Indicators we use to determine level of intensity are breathing patterns, sweat production and heart rate. When these measurements reach a certain level for your age group and level of fitness, then you enter what some refer to as the "cardio zone". This simply means that the body's mechanisms for dealing with a stressful situation has begun to demand more out of your heart than what it is typically used to. This is the basis of muscle development. The heart (a muscle) must be challenged. Your goal here should be to maintain that intensity for about 5 minutes per session. You should have 3-6 sessions per week.
The great part about this method is that you can work as quickly or slowly as you want to reach the right intensity. And as you progress in your fitness, the rules don't change. 5 minutes of intensity will get the job done. But you will find it harder to reach the desired level of intensity as your heart becomes stronger.
As for minimums/maximums etc: The heart is perfectly capable of staying healthy and strong without ever reaching these levels of intensity. However, in our present society with so many poor choices of food out there, it is a good idea to supplement your health habits with good quality exercise. I think 5 minutes of elevated heart rate, sweat producing activity 3 times per week would be the minimum goal to shoot for.
When does it become overkill? That's a bit more subjective because the heart can and will continue to improve it's performance capability with increased demand. I'd say just do as much as you're comfortable with and provided you're reaching your intensity goals, be satisfied knowing that you are keeping your heart healthy and strong.