As long as you enjoy practicing it. This is one technique (unlike many other yoga practices) that cannot be overdone in the sense of putting a strain on the nervous system. Yoganandaji used, as a boy, to practice it as much as 7-1/2 hours at a time. He once told a disciple that if one wants to become a master in this life, he should practice Hong-Sau two hours daily. No technique, however, should be practiced to the point of boredom or fatigue. Beginners, especially, may do better to practice only half an hour at a time, perhaps even less. For others, let enjoyment be your key, lest you slip gradually into the pernicious habit of meditating mechanically, without that keen sense of blissful anticipation which is so necessary to any real meditative progress. When your enjoyment of the technique begins to lessen, cease your practice at least for that session. When your enjoyment of meditation itself lessens, stop meditating, or take a break (you might rest in Savasana (The Corpse Pose) before making another effort.