A common question from novice lifters is "how many times should I train a muscle group per week?"
Many people are under the impression that if they don't give a muscle 168 hours to recover, it will get a bad case of over-training syndrome. It will begin to fall off of their body striation by striation.
OK, so they may not truly believe that last part (I sure hope they don't).
Ask any weekend warrior about their training split and I am 80 percent certain they will spit out a five day bodypart split that they came across in last month's Muscle and Whatever magazine. Will training each muscle group with such a low frequency provide results? Of course, but most people will find they have more optimal results if they jump in the squat rack a few extra times per week.
How often to train each muscle group is highly individual, but I do feel the majority of natural athletes should train every muscle group no less than twice per week.
Training a Bodypart Only Once a Week?Whether you realize it or not, when you perform any weight-bearing activity with the hopes of getting stronger or building more muscle mass, you are aiming to maximize the effects of supercompensation. In short, supercompensation is a relationship between work and rest that leads to superior physical adaptations. 
The phenomenon occurs when you place a stress on the body that generates fatigue (ex: weight training) and you allow the fatigue to dissipate with adequate rest. The body adapts (more strength, endurance, speed, muscle mass, etc) and surpasses its old homeostasis (resting state/baseline).
So how does this relate to training frequency?
Well the "post-workout eating window" may be complete BS, but there is a real window for maximizing the physiological adaptations that result from supercompensation. The peak of the supercompensation cycle is 72 hours. At that time, force generating capacity and muscle soreness has returned to baseline, psychological supercompensation occurs, and glycogen stores are fully replenished. 
The more time that passes between the peak of the cycle and stimulation of the same muscle group, the larger the decrease in previously acquired adaptations. However, if training resumes around the 72 hour mark and progressive overload is present, a trainee can build upon the new adaptations and set a much higher homeostasis.
Many people have gotten good results from training a muscle group once per week. But I think they will have better results if they provide a new stimulus before the adaptations from the previous training session begin to digress.
Factors Affecting Training FrequencyDespite my belief in a higher training frequency, I do realize that some people need to go below or above my recommendation. Muscles grow while at rest, not in the gym.
Sufficient recovery is needed in order to train each muscle group more frequently without causing negative adaptations and decreasing performance.
Factors That Affect RecoveryAge: Although there are exceptions to the rule, young children and older adults generally need more time to recover in between training sessions and benefit from training each muscle group less often. That is not saying that they need to give their muscles an entire week to recover, but having an 80 year old man try to squat heavy five days per week is probably not the smartest idea.
Training History: The more advanced a lifter you become, the more volume, intensity, and frequency needed to stimulate progress.
A novice lifter will still see positive results from training legs once per week (not saying this is necessarily optimal), and possibly need the entire week to recover from the training session. An advanced lifter would most likely see little to no positive adaptations, and the amount of plateaus he would run into would probably have him in a mental asylum within two training cycle.
Sufficient recovery is needed in order to train each muscle group more frequently without causing negative adaptations and decreasing performance.Sleep and Stress: Sleep is extremely important to recovery and I would argue that lack of sleep has a more noticeable negative effect on performance than poor nutrition. Failing to get quality sleep will negatively affect recovery and force an athlete to wait longer to hit the same muscle group again.
Excessive stress is another major blow to the recovery process. If stressed out of your mind, attempting to maintain a high training frequency will lead to stagnation and regression.
Nutritional Practices: If you are not eating properly, you are not going to recover from your workouts, period. Food is fuel.
Failing to adequately meet macronutrient and micronutrient goals is like trying to drive your car without gasoline; it is impossible.
How to Train More FrequentlyIt is really not hard to begin training each muscle group more frequently, just perform the necessary exercises more often. However, trying to immediately jump from squatting once per week to seven will have you riding around in one of those fancy electric scooters.
There needs to be some strategy to your programming, and unless you are doing a specialization program for a defined period of time, it is generally wise to give each muscle group around 72 hours of rest before hitting them again. There are a few simple templates for three, four, and six day training splits.
If you only have three days to lift, hit your full body on all three days with an emphasis on a different main lift each session. If you have four days to devote to training, train the lower and upper body twice per week. If you are in the gym six days per week, work pushing movements, pulling movements, and legs twice per week.
Experimentation is the only way to find what works for you. There is no "perfect" formula for designing a program. The most important aspect is to make sure the training variables you manipulate (exercise selection, volume, intensity, sets, rest, tempo, etc.) are specific to your goals. Once a solid foundation is established, there is no limit to the creativity that can be placed in to your periodization.
Summing UpI spent a descent amount of time training each muscle group once per week, and my results were far from stellar. It was not until I spent anywhere from two to five times per week in the squat rack, deadlift platform, or flat bench that my gains started to soar.
Not only do you maximize the effects of supercompensation when you train each muscle group more frequently, but you greatly increase your exercise technique. Lifting is a skill, and skill development is a direct result of practice.
I will leave you with a quote from Mike Zourdos, a professor at Florida Atlantic University. Dr. Zourdos said:
"If someone took your family and the only way you could save them was to put 100 pounds on your squat in a few months, would you only squat once per day? No! You would squat every chance you got."Do not take things to an extreme, but experiment with various training frequencies, find the one optimal for you, and your gains will take off.