When is too much, too much? Well physiologically speaking the Central Nervous System can only handle so much. Just, think all professional, collegiate and amateur athletes work based on a periodized program design. Why, simple put there is a natural intelligence in which your body operates with and violation of this will lead to eventual things such as injury, a decrease or poor progression, burnout, etc.
Your CNS, brain, muscles, joints, ligaments, tendons, organs and hormones can only handle so much and when you consider how poor most people eat, the lack of sleep and stress that accompanies the typical or average person you can see that overtraining and constant high intensity, high output work is not the most intelligent way to train and for that matter treat/care for our own organism.
Are you performing at your peak, are you aware of the signs of over training, are you being deceived, what do you know about the physiology of human performance? Ask yourself these questions as you design your program and train your body/brain/human organism
I can go on but these simple physiological facts and questions are meant to help you take a deeper look at your practice of optimal living/performance.