Padmasana facilitates relaxation, concentration and ultimately, meditation. The posture creates a natural balance throughout the body/mind. When the knees are stretched enough to remain in the padmasana without discomfort the posture creates a feeling of effortlessness and ease that will soothe the nervous system, quiet the mind and bring about the concentration

In the course of a typical asana routine you might hold it for several minutes or until you experience discomfort in the legs. When used as a meditation posture you hold it for the duration of the meditation.