We were talking about the connection between one’s physical and mental performance and found that exercise and fitness definitely affects mental performance.  With regards to duration, however, it appears that improved mental performance is apparent after a “warm-up” period, leading up to a moderately intense level of exercise (about 75% of your max heart rate), and lasts until physical fatigue.  Fatigue, of course, is based on an individual’s fitness level.  Once fatigued, mental performance could actually decline to a level lower than when you started!  Worry not, it’s only temporary!  The reason for this improvement lies in the increased flow of blood and stimulation of neurotransmissions.  Note that the ability of your blood to carry oxygen, nutrients, and fuel to the brain, thus improving brain function, may be improved through cellular cleansing, but that is a topic for another time.  For this discussion and in general, the more blood, the better.
Though physically fit people tend to have greater motivation, eat healthier, and are generally more alert and engaged, there is no evidence that fit people operate at a higher level of mental performance in between exercise sessions.  Dehydration, on the other hand, in alignment with the point made in my previous post, is directly connected to a notable reduction in mental performance, whether it is exercise-induced or caused by other factors.  Luckily, rehydration will return cognitive function quickly, especially if sugar-containing fluids are consumed rather than no-calorie fluids.
In the short run, each session of aerobic exercise has the potential to give you a double benefit for your time spent. Not only will you be improving your fitness, your ability to concentrate on and perform mental tasks also will likely be enhanced.  In the long run, physical activity appears to be at least as important in staying mentally sharp as keeping your mind active and maintaining strong social connections. Multiple studies have shown that people who exercise regularly will have less age-related mental performance degradation and lose less brain tissue seen on MRI and PET scans.