Sorry man, but I don't buy dat hyperthropy rep range of 6 to 12 reps. I ALWAYS made my best size gains not going much more than 6 reps. Many say the set should last somewhere from 30 to 90 seconds, and I say BULLSHIT 
I think sets shouldn't last much longer than 20 seconds. Also, as far as "extending the set goes" some say that doing so creates too great of an "inroad" into recovery ability.
probably because that is when you were getting the strongest. load is the most important factor, but time under tension is second. so youl get biggest fastest by getting strongest fastest, = lifting lower reps/heavy weights. but at a certain point strength gains start to become very difficult, once your close to your complete strength potential (lets say benching 500lbs raw), and at this point your probably better off trying to increase the amount of reps you can get with the heavy weight, instead of tryinng for further increases in workload. thats why i said guys should focus on powerlifting untill they lift big weights, and then focus on adding reps/sets while using the big weights.
recovery is always of the essence, its a personal matter i think, one youve got to work out on your own. a wise man once told me, the first set you do where intensity/strength has decreased, thats when you start over training= once strength has decreased, youe done as much as you can do in one workout.
my theory on overtraining is that the muscle responds to load and tension, but both of those can cause muscle damage (aka microtears). the more microtears you have, the harder it will be to recover. you want to maximize the growth stimulation with load and tension, while making sure that your not fucking up the muscle too badly. usually, at least in my own experience, the muscle will get weaker and feel a bit numb once ive reached a point in my workout where i would be overtraining to continue. rest time between sets, whether or not im training to failure, total weight(rep range), and what ive eaten before hand all effect how long i can train before overtraining. when i take long rest sets, stay away from failure, and have some carbs with my pre workout meal i can train longer and get better results.
one more thing i would add.. once youve reached the point in a workout wher eyour muscle is weaker, and you would be overtraining if you kept lifting hard... you can still do a few light weight/high rep pump sets and this wont be overtraining, so long as you stop them shy of exhaustion. and doing this helps with recovery and growth.
