The mastery of the full form can take anything from 3-7 years, it all depends on the individuals capacity. One should never go beyond one's limit. By exceeding one's limit one enters a zone of danger and potential of injury. This should be avoided through the exercise of patience at all times.
The essence of Yoga practice is learning how to avoid or overcome obstacles without rupturing the laws of nature. People of poor health should seek guidance from thier physicians before attempting the practice.
The purpose of yoga is enlightment through discriminitive wisdom. It manifests to diffrentiate the soul from all that is not the soul.