Ok since i feel i'm a prime candidate for the subject of "over-training" i'll shed some of my experience with it to help you out!
First off, i don't believe you can over-train entirely BUT there is a point where you will need a few days off to recover/grow. ANYWAYS i have been busting the crap out of this "myth" for the past 5 months. I believe that with the intensity of the training reflects how much more you should be eating to FUEL yourself and the increased metabolism resulted from more frequent/longer training sessions.
Currently i train Full body 3-4x a week like this.
Day 1: Full Body (Chest/Back PRIORITIZED) That means i start with chest/back.
Day 2. OFF completely
Day 3: Full Body (Legs PRIORITIZED)
Day 4: OFF completely
Day 5: Full Body (Shoulders/Arms PRIORITIZED)
Day 6: OFF completely
Day 7: Either OFF depending on how i feel or i will do weak point training PRIORITIZED along with full body
Sessions average approximately 3 hours and i consume a BCAA/Carb drink (like Size-On or something of the sort)
I also consume around 1-1.5 gallons of water and have energy through the entire session (99% of the time)
After a few weeks (being around 3-4) i may take a few extra rest days and consume higher calories.
You can doubt this training all you want but there's a reason why the guys back in the day trained this way for a few reasons i can think of being:
1. Burning more calories (Less reliance on boring cardio)
2. Better cardiovascular conditioning (this is due to super-setting two different exercises for different muscles and taking minimal rest)
3. Faster strength and size gains (Depending on your calorie intake VERY IMPORTANT)
4. More practice with exercise movements.
5. Improved Muscular conditioning.
These are the effects I HAVE experienced, the biggest determining factor that will also determine your progress is the CALORIES. (The highest point i was taking in 6-7k calories for a few days then dropping back down to 5-6k) You cannot eat like a RABBIT and train this way effectively! Who knows though this may work for some and others may not work so well but it will almost always boil down to intake of food. Also when you start feeling very run down through sessions is the point where i would take a few days off until you feel 100%.
Here's my older training log:
thetinyguy's ALL OUT training log!! *CAUTION OVERTRAINING*
When i first began trying to gain weight i made 2 choices: To train as hard as i possibly could and to eat as MUCH as i possibly could. I believe in high calorie/super high intensity training. During that period i trained 3 hours with a Chest/Back, Legs, Shoulders/Arms split and it worked really well i went from 170-240lbs over a winter and proved to myself what i was capable of. I went of the beaten path and tried something that seems to have been buried in the modern training world. Arnold and the golden era guys trained that way for a reason.
Hope this helps you out man! Impossible is nothing and anything is achievable.