teachings from a master on mastery

T. Krishnamacharya, November 18, 1888 – February 28, 1989, is considered the father of modern Yoga. 

This is an excerpt of a teaching from Sri Krishnamacharya to his student Srivatsa Ramaswami:

When one starts to learn Yoga, in the beginning the duration of
practice can be as little as 15 to 20 minutes. One can gradually
increase the duration. The teacher should check the breath every day
and then increase the duration of practice. Whatever be the posture,
if one could stay for a long time without the limbs going to sleep (or
numb) or any pain or discomfort then such a practitioner is known as
jitasana (the conqueror/master of an asana.) While staying in an asana
one should not unnecessarily shake the body, bend or contort or move
and if one can stay for hours then such a yogi is a jitasana. We learn
from the works and sayings of yogis that in the olden days the rishis,
every day would remain in any one asana for three hours and do
pranayama and meditation. Then if the yogi is able to remain doing
long inhalation, exhalation and kumbhaka without feeling any kind of
fatigue and for a long period of time such a person would be called
“Jitaprana” or Jitaswasa, or one who has conquered the breath.