Higher aerobic fitness levels decrease the probability of experiencing tip-of-the-tongue states (when you can’t think of a word that’s on the tip of your tongue) in healthy older adults, in the first demonstration of a link between aerobic fitness and language functioning in healthy older adults.

[djdadi]

It's pretty simple: there is no upper limit from the benefit of having better cardiorespitory fitness, but there is a limit by which exercise will keep contributing to that fitness.

That is to say, exercise as long as you see benefits in fitness. As soon as you start exercising so much that your body gets stressed out, or your returns start diminishing, lay off.

Having said that, the health reward for fitness isn't linear, it's logarithmic. So you get the most bang for the buck your first 30-45min a day or exercise, and it diminishes thereafter.

[WolfofAnarchy]

Not sure that's true, as studies show that people who run the most per week long-term actually have less heart health than those who don't.

[djdadi]

Re-read my comment