To begin with , the practitioner should stay in headstand position for 10 seconds and gradually add 5 to 10 seconds each/alternate day so as to reach a stage when he/she can hold position comfortably up to 3 minutes. The 3 minutes stay may be continued for weeks and after than gradually 5 to 10 seconds can be added to reach up to 5 minutes.
It may however be noted that each individual is unique and the minimum or maximum holding time is likely to be different for each individual depending on his/her physical/medical profile.
In certain conditions like back injury, headache, heart condition, high blood pressure, menstruation, neck injury, low blood pressure, pregnancy, glaucoma, detached retina, hiatal hernia, obesity, conjunctivitis, brain disease, brain injury etc. it is not advisable to practice headstand.
Also this asana should be practiced under supervision of an experienced teacher or after accumulating sufficient prior training/experience only.