Hey guys.
This is an important topic and very well worthy of a good reply. Let's see what I can do to clear the issue.
HST advocates training a lot less longer than traditional programs. This is because we also consider the major role played by cortisol and the whole process called
gluconeogenesis.
Elementary info we all know: as we train and exert all the herculean effort we do to lift weights rep after rep and set after set, we exhaust our body's energy. Glycogen (in muscles) and glucose (in the blood) are depleted.
No surprise there, we basically knew all that.
But as glycogen and blood glucose levels drop, cortisol level rises. Cortisol is a catabolic hormone that the body produces. Too much cortisol (and letting it run rampant in your body) will literally destroy all potential gains, and can even devour some already hard-earned muscle. Cortisol literally converts muscle tissue to proteins for conversion into glucose. This is your body’s way of producing energy when all readily available energy (glucose) and stored forms (glycogen) of energy have been expended. To compensate for this depletion of energy your body will go into a process called gluconeogenesis to produce glucose from amino acids in the liver.
This is why we eat after working out, or drink a post-exercise shake, so we put a stop to rampaging cortisol and enable protein synthesis to do its job.
This is why we generally advise shorter workouts against longer workouts. Because if you really go at it seriously, no chit chat, and you push yourself to your limit, then you seriously deplete your energy. And the longer you do that, the more cortisol is produced to counter the energy depletion. Our body isn't exactly a genius, so even though we are working our butts off to create muscle, he goes
"I detect energy depletion, okay, I'll produce cortisol and cannibalize some muscle to produce more energy."
So how much cortisol also depends on how quick you actually deplete your energy stores. If you are panting and feel like dead after 15 minutes but still have an hour to go, chances are by the time you finish your workout, even if that's just an hour and 15 minutes total, cortisol probably cannibalized an unholy amount of muscle just to get you through your workout. Pretty much beats the purpose of it all, huh?
Of course, that scenario is too much of an exaggeration. If you feel like dead just after 15 minutes, then you obviously have to change your routine, or perhaps something is wrong with your diet.
Now, based on all that, let's go back to the "just how long" question.
There really is no written scientific law about that. I'd say 45 minutes to an hour. Some say just 45 minutes. Some say a little over an hour. Whatever you follow or decide to do, just remember that it is pretty much an issue of having enough energy to last (most of) the workout, so your body doesn't depend on muscle-and-protein-eating cortisol just to get you through the workout.
So even if you work out for two hours, but you make sure you somehow get yourself enough energy to last through most of that (like you ate well 2 or 3 hours before, engaged in no heavy activity, drank a good serving of a high-quality pre-workout shake, and perhaps even took a minute or two in the middle of your 2-hour workout to drink a carb&protein replenishment drink or whatnot), then even that 2-hour workout wouldn't be so unholy. But accomplishing that is tricky business. So the simpler and more effective way would be to make sure we don't workout too long, and we eat or drink a post-workout shake immediately after training.
So try to make it as short as possible, which includes possibly dropping an exercise or two that you can do without. But even if you can't put your workout under a certain limit, say 45 minutes or an hour, as long as you are doing your best, are reasonably sure you are doing just the right number of exercises, and pretty much have enough energy to accomplish the workout and have a meal or handy post-workout shake ready right after you train, then you don't have to worry at all.
Hope that helps. Sorry it's so long. Happy lifting!

-JV