What is the correct intensity during training?

Training can be carried too far, which leads to an unhealthy state. Everything must be done in moderation, as your system needs to get used to gradual development. Start slowly and make gradual progress.
Never force movement on yourself, and never torment yourself with exercise that goes beyond your capability, because you might get hurt! Too little work is not effective, too much is very dangerous.

The intensity of training shouldn't be so high that at the end you are struggling for air, or have to stop. As you know, during a lack of oxygen, muscles get tired more quickly, and on top of that fat is only burnt off in the presence of plenty of oxygen.

The most important thing is how you accomplish the exercise. Do as much as you can: concentrate on correct performance, since if it is not performed correctly it's not worth a thing!
Your heart and soul determine your real top intensity limit, but you need to sensitively concentrate on your muscles during training. Your mind is capable of anything. Even if you were completely exhausted, and all your power abandoned you, you could lift a heavy weight if you concentrated on it hard. But avoid this situation, because it can easily lead to accidents. When your movement falls apart and you cannot perform the exercise correctly, it is a warning of your momentary top limit: stop immediately.

You always have to train beyond a pleasant feeling, to make your body to adapt. Even if you are running out of power you are able to finish the training method, if you want it a lot. At this time, think: "I can do this for great results and advantages in future! It's much better to live a powerful, balanced life than a weak and sick one! I will follow through the training!"

Your system and muscles need to relax at least for a day between the same type of training. The total recreation time of your muscles is 48-72 hours, so there is no point training the same muscle within this time frame.
If you always keep to this relaxing time for your muscles, you will reach a sustained result. In addition, you will keep the risk of injury to a minimum, since it is more difficult to direct a tired muscle, than a relaxed one.