>>5813849You can do them for many things, depending on your definition of study.
For some people, a 3h head sketch is a study. For some people, a 20min 2value sketch is a study. Both are valid, both will teach you things, different things.
For portrait studies to be most efficient, you need to do them with a clear, precise goal in mind. As an example, "getting better at portraiture" is a nice goal, but it may be better to break down that goal in smaller goals, like: "arranging values", "those darn noses!", "how to light a face?", "training for proportions", etc.
Then, you should be able to understand, for each such goal, what would be the most helpful way to study.
