The main thing to consider when balancing your training is recovery. Recovery from training takes place on two levels: physiological and neurological. Both systems are taxed, and while there are physical processes that contribute to recovery, the central nervous system must recover as well. Training with heavy weights places enormous stress on the central nervous system, even if the same muscles aren't worked two days in a row.
Physiological recovery involves replenishment of nutrients in the muscle cell and repair. Nutrients include glycogen, the fuel for the cell, and regeneration of ATP, or adenosine triphosphate, the actual "engine" of the cell that produces contractions. Cell tissue that is damaged must be repaired. Hormones are released as a response to training and perform various functions.
The central nervous recovers in other ways. The "pattern" of the exercise is imprinted and the body learns to become more efficient at that movement. This is one reason your strength can go down if you perform the same execise workout after workout because the body becomes so efficient that it stops responding- you overtrain the movement pattern. Certain complexes in the body, such as the Golgi tendon organ (GTO), have parameters that are designed to protect the muscle from injury. The nervous system also becomes more efficient at firing groups of motor units (the building blocks of muscle cells), which translates to a strength increase because the muscle is able to generate more force output - this is known as recruitment.
For these reasons one of the most popular methods of training is to alternate resistance training sessions with cardiovascular sessions. By balancing cardio and training, sessions can remain short, yet intense. Several days pass between working a particular muscle group, allowing for physiological recovery, while the cardio day allows for the central nervous system to recover from the prior resistance training session.
This is not to say that other popular methods, such as 3-on and 1-off, whereby resistance training is performed three consecutive days in a row and then skipped for one day, are not effective. Each person has a different capacity to respond to training and recover, so it is important to try various methods and determine which works the best for you. For example, it is very popular to train the muscle twice or more in a week. Often, one workout is high intensity (expressed as percentage of 1-rep maximum load) and the second workout is lower volume and/or intensity to further stimulate the muscle while still allowing sufficient recovery.
Something else to consider when balancing training is the total workload. Workload can be defined in many ways. You may consider the volume of the workload, or how many repetitions of work are performed. You may consider the intensity of the workload - that is to say how heavy the weight moved is. Intensity is also affected by time under tension (TUT), or the amount of time that a particular muscle is required to contract. A 4 second repetition will place the muscle under tension longer than a 2 second repetition.
A short, heavy workout would be considered high intensity and low volume. A marathon run would be considered low intensity but high volume. Intensity in this situation refers to the weight lifted - not the perceived effort. Obviously, a marathon is going to require a very intense effort, but each stride is of a much lower intensity.
Training should take these definitions into consideration. If you are training for a marathon, then a 3-day resistance training and 3-day cardio regimen is most likely not optimal - marathon training will require you to perform cardio up to 5 or 6 days per week. In this situation, you must recognize that you are performing low intensity but high volume work with your lower body and core muscles. In order to balance your resistance training, you would want to stimulate your lower body muscles with less volume but higher intensity work.
On a side note it has become common practice in recent years to move a weight slowly and 'feel' a muscle working. The upshot of this is that your typical gym instructor wil say the weight doesn't matter when developing muscle size and strength (ironically your typical gym instructor looks like they have never lifted a weight in their life!). The truth is a slowly moved weight will work your endurance based muscle fibres and little strength will be developed.
If you are looking for muscle size then you can forget it with slow repetitions. Move the weight as quickly as you can on the positive or actual 'lifting' part of the repetition and avoid ballistic movements- but no momentum or you may get injured. You may go slower on the negative or 'lowering' part- try to resist the weight on the way down. You will move more weight in less time- and anyone who knows basic physics will know this means more work has been performed. You will also get stronger and more defined muscles!
However you train it is a good idea to reduce the intensity every 4-6 weeks. Due less worksets- maybe 3 instead of 8 or reduce the weights by 40%. You may find you come back stronger the following week- you will certainly have more energy and be more hyped up for your workouts!