What limits? The point at which continued training starts being painful (not in the "feel the burn" way but "something's gonna tear" way)? The point at which you can't even lift a light weight anymore? The point at which injury occurs?
Training works by disrupting "homeostasis", which is an equilibrium for your body. It is the state in which it resides, untrained, and towards which it will return if training is halted. Your body will adapt to new stresses imposed on it, but if that stress is removed, the adaptation will not last because maintaining it is physically expensive. An organism's body is like a self-tuning machine that wants to be functional and use its resources efficiently.
Getting stronger and getting more muscle tissue will always take this form, no matter what your training level:
- Do something that disrupts homeostasis. For example, lifting weights.
- Recover and let your body adapt to its new situation.
- Repeat from 1, but with a stronger stimulus.
- Once a situation is reached that is satisfactory, maintain it by applying the same stimulus over and over.
Number 3 is important because, if you were to keep lifting the same weight, or keep using the same volume (reps, sets...) your body has adapted to it and it no longer is a stress that urges it to improve. By creating a stress, recovering from it and letting your body adapt, you'll now have a new baseline. From there you can impose a greater stress, that will repeat the cycle. Number 4 is important because if you were to stop training once you consider yourself strong and/or muscular enough, your body won't feel the need to keep all that around and you'll slowly regress to your pre-training state.
So in that respect, making any improvements at all will require that you pass your "limits", if your current baseline is considered your limit. However, that's usually not what we mean by "limit". Passing your "limits" is impossible because of the definition of a limit, or it means you've gone beyond some point that's within common sense and you'll suffer injury.
So the question then is, exactly how much stress must you impose? Judging by your question, you have a "more is better" mentality. That isn't true. You must induce enough stress to disrupt homeostasis, and also enough for the adaptation to be meaningful. But not so much that it can't be recovered from.
Say that you go into your first gym session and you start squatting. After getting technique down you start with the bar, and then do sets of 5 reps with 5 kg / 10 lbs increments. At some point it starts feeling heavy, so we see if an additional 2.5 kg / 5 lbs is manageable. If so, that's a good starting point. If technique starts suffering, maybe we take a little weight off and then do 3 to 5 sets (depending on program) with that weight. This is enough to disrupt homeostasis, but can also be recovered from. The rookie lifter will come back into the gym 2 days later and be able to do the same number of sets and reps with 5 kg / 10 lbs or 2.5 kg / 10 lbs more, and this will continue for a good amount of time, because the rate of stress, recovery and adaptation are right.
Now if we take our beginner lifter up to a weight that feels challenging on the first workout, and then start screaming "BREAK YOUR LIMIIIITS!" and add another 5 kg / 10 lbs to the bar, and have them do 10 sets, no matter how much technique suffers and how slow the movement back up becomes, you'll either set your lifter up for great injury, or when they come back 2 days later they're barely able to stand up, let alone do another workout.
A martial arts champion will have started as a rookie one day. If we take a rookie and put him in the ring with the champion and tell them "go nuts, boys", the rookie won't make quicker progress. He'll go to the hospital, or morgue. The rookie must start with rookie training, in the right dosage, to make progress. Then maybe one day he'll be a champion. Like coach Mark Rippetoe has stated a number of times: you don't get stronger by lifting weights, you get stronger by recovering from lifting weights.
6 hour workouts seem rather absurd if you're not a pro athlete, not a pro bodybuilder or on a ton of steroids. If you're a skinny guy, doing Starting Strength, StrongLifts 5x5 or some other sensible beginner program without any extra crap, eating enough and sleeping well will make you gain strength and muscle. And what's more, it'll be at a close to optimal pace.