Exercising more than 45 minutes is beneficial and makes a person more fit exercising for more than 45 minutes is a waste of time. If you are not exhausted after 45 minutes then you are not working hard enough. Growth hormone levels begin to drop the moment you begin training. It continues to deplete at a fast rate during the workout. By 45 minutes growth hormone levels plummet. When growth hormone levels drop too low cortisol levels increase. Cortisol inhibits muscle cell receptors ceasing any further fat or sugar metabolism. In other words, you are wasting your time training and only increasing the risk of injury or catabolism.
Daryl Conant, M.Ed