In Hatha Yoga books it is said that a particular Asana is considered to be mastered when the sadhaka can maintain it steadily and easily for four hours and twenty minutes. But this has really no important significance. It gives merely an approximative idea of the length of time for which practice may be undertaken for gaining mastery. Since the Asana-anga is part of seven angas, physical exercises are not an end in themselves. Pranayama , control of subtle energy.
Breathing is only an aspect of it.
The practice of real Pranayama cannot be undertaken until and unless one of the asanas has been mastered. The advanced practice of Pranayama arouses the Kundalini sooner or later. Pratyahara , withdrawal of the senses of the mind from their objects. Dharana , confining of the mind within a limited mental area. Dhyana , uninterrupted flow of the mind towards the object chosen for meditation.