Ling Shu Ch 15 delineates the warp and woof of heaven, earth, and humanity:
The 28 constellations correspond to the 28 primary meridians. The total length of the 28 meridians is 1,620 cun. Ying qi travels 6 cun in the meridians per respiration. Therefore, it travels one complete cycle in 270 breaths. During that same period of time, the earth travels 1/50 of the distance of its circuit around the 28 constellations. In one day and night, the earth completes one cycle around the 28 constellations, while ying qi completes 50 cycles through the 28 meridians.
While the ancient astronomy here may be abstruse, the intention is clear. It is a total integration of heaven, earth, and humanity. Events in these different realms correspond with each other. They occur together in a tightly woven matrix. With every breath we express the quality of this integration .
Since ying qi circulates through the meridians 50 times a day, each cycle takes 28 minutes and 48 seconds. It makes sense to allow for a complete qi cycle in an acupuncture treatment. So, we can retain needles for about 30 minutes. Indeed, many acupuncturists, including Master Tung of the ‘Miraculous Points’ fame, use this as a guide for needle retention time. But that’s just one approach.
Ling Shu Ch 12 specifies different needling depths and needle retention times for each of the 6 foot channels. Points of foot-yangming, for example, should be needled 6 fen deep and retained for 10 breaths; points of foot-jueyin should be needled one fen deep and retained for 2 breaths. Prescriptions for the other four channels fall in between. Points of all hand channels should be needled no deeper than two fen and retained for only one breath. Furthermore, these prescriptions should be modified to fit each person’s condition such as size, height, and age, as well as the nature of the imbalance or disease.
So how long should the needles stay in? The question deserves consideration on an individual basis. Everything in an acupuncture treatment matters: it can make the treatment more effective, or it can make the treatment less effective. In any case, since the purpose of adjusting needling depth and retention time is to elicit the ‘arrival of the qi’, we can say that when the qi has arrived, the acupuncture treatment is done. But that is another story.