For a long while I've know that 45 minutes per session has been touted as the optimal recommend time in the gym for a heavy weights session. What I didn't know was why 45 minutes. I had for a long time thought it was because the body depletes itself of testosterone and the "good" energy and after 45 minutes every minute is a case of diminishing returns.
I read an article today that reported something slightly different; something that in light of what else I know makes a whole lot more sense. After about 40-45 minutes of intense exercise the body starts to produce cortisol thanks to the stress it's under. (Cortisol is produce by the body during times of stress. Not just weight training stress but other stress like emotional stress).
Without boring people with long words, after that 45 minute mark the body cortisol build up results in a release of some stuff that provides the body with temporary relief from pain. However, if this goes on for too much longer this causes important (to body building) functions to come to a screeching halt (the same thing happens when doctors supply patients with corticosteroids). It's the maintained period of high intensity that does us in.
Muscles grow while you are not in the gym, while you are resting and too much gym time, even if only 45 minutes at a time, leads to overtraining and over production of cortisol, which leads to a whole other bunch of issues I didn't go into detail about like: faster aging, excessive amounts of free-radicals, and damaged DNA.
Over the years I've tried a few different training schedules and I've found that working each body part only once a week for about 45-60 minutes per session works the best for me. Most of my sessions are around the 45 minute mark, but sometimes I go over, usually if someone stops to talk to me (grrr) or the benches and weights I want are in use.