I have 2 things to say about that. The first one is that depending on what your goals are, if it takes you longer than one hour 5 or 6 days a week to train then you're probably resting too long and training too light.
The second thing is that when you train you want to maximize testosterone and HGH while minimizing cortisol. The shift from test to cortisol tends to happen about an hour after you start training. This is a simplified explanation of a complex reaction, but that's why most people go by the 1 hour rule.
Personally, I'm spent after about an hour and it's useless to push it any further. Hit it hard then head on home.
Current PRs at 242- 495, 345, 585, 1425