What about Olympians? They have a pretty harsh schedules......it is all about how you train, body composition, gentics etc..
You can train your body to handle anything even workouts for hours!
I currently train 6 days a week-3 days on then rest/repeat, hitting each body part twice a week, but each workout is less then if I was doing each body part once per week (so 2 smaller workouts for each body part as to 1 big workout for each body part). I will train like this for a couple of months then switch it to 4-5 days a week hitting one body part each week-I do not show any signs of over-training and feel stonger with each workout-on or off cycle.
I have a friend who trains martial arts 2-3 (at least an hour each session) times a day-every day plus his weight training 4-5 times a week and no over-training, he eats, takes the vitamins and minerals he needs and rests according to his training-it is all how you know your body and what it can handle and what you can make it handle!!