Is once a week enough? It depends.
ALL of the fitness models and bodybuilders don't just work one body part per week. Their programs vary, as should anyone elses, based on body, muscle fiber type and training routines.
I have a friend who could model in anyone's fitness mag and she works glutes at least 4 times per week. Once a week for my lower body wouldn't get it for me, either. But, although we have more frequent training routines in common, our body types are opposite and the reasons for our routines working for us are vastly different. How frequently you should train a body part depends on the individual.
While there are a variety of factors that play a role in increasing muscle fiber size (hypertrophy), there is one MAJOR factor in the capacity of the muscle as a whole to increase in size--the total number of myofibrils--and, according to muscle
physiologists, that number is fixed by the second trimester of fetal development, in other words, genetically determined. It is independent of sex, training, predominant fiber type, nutritional status after birth, hormonal levels, etc, since
the current thinking indicates that humans do not have the capacity for muscular hyperplasia (increasing the NUMBER of fibers) after birth.
Fiber type and hormonal levels determine potential for increasing the size of an individual fiber. Progressive overload and nutritional status determine the degree
to which each fiber reaches its maximal size potential. Volume of training determines the degree to which the muscle reaches its overall potential for growth (maximizing size of all fiber types). But the primary factor for determining how
large a muscle has the potential for growing is dependent on the total number of fibers--something completely beyond our control and the destiny of genetics.
The point of the previous paragraphs are to indicate that there are no hard and fast rules. Following the training routine of a person with a different body type, with muscles more or less dense, and/or with a different predominant fiber type than you won't work the same for you as it does for the other person.
Frequency will be an individual thing based on all the above and depending on your goals.
Maribeth