Yoga is a very varied way of exercise -- e.g., doing Surya Namaskar many times will of course improve one's stamina; most of the yoga asanas will build both strength and flexibility; some will help to develop a better sense of balance.
(Compare these questions: "What are the health benefits of Yoga?" and "What are the best benefits of yoga (versus cardio and weight training)?")
As mentioned in the answer to "Which daily exercise are helpful to maintain health for long?", there are certain recommendations the American Health Association gives regarding how often one should work on certain aspects of fitness, like moderate cardiovascular training should be done almost every day (5 days a week), while high-intensity muscle-strengthening is recommended only 2 times a week.
How does yoga fit into that picture?
Considering that yoga is in itself a kind of "cross-training", how often and how long would one do yoga asanas, in order to improve overall health in the sense the AHA recommends?
In what way can one's yoga practice be adapted to meet such recommendations, for instance by leaving some strengh-intensive asanas out for a few days, especially if yoga is practiced on a daily basis?
More specifically, are there any studies or otherwise noteworthy recommendations on how to arrange one's yoga practice in order to improve overall long-term health (compared to just getting better at doing the yoga exercises as an end in itself, as discussed in this question)?