good morning-
I go by numbers, rather than time. Going by feeling, as Margaret mentions is also good. I do six times. Nine is a good number if you want to do more. Once, after the active set is enough. Partial versions , here and there through out the day is ok. I often twist my trunk, and keep my spine open. It doesn't take much to tighten up there. This twisting of the spine, and the bending over and rolling up of the spine are extremely important. There are whole sets of qigong exercises, that just work the spine, pelvis, and hips. It opens the verbrae and joints that close with age, or, are always somewhat closed or closed, throughout your life. It lubicates the spine.
Any spine work is very good, anytime. Spend some time making those spine rolls make sense and feel right. It should feel liberating. Use your third eye to help you roll up one vertebrae at a time, all the way up through the C7, base of the head, and crown.
This is a great movement, very important. Along with rolling and twisting the spine, a gentle swaying side to side is good(like a willow in the wind), opening the spine in that direction. Horizontal circling the spine at the base, middle, and shoulders, both directions, about softball to sm melon size circles is another. Moving up and down the spine with that. If you do one of these circles at a time, moving up the three areas in sequence, it becomes a spiral movement. The spine is very important, and those spine movements in harvesting qi are very important. Movements should be fluid. Probably, the hula would great for the pelvis!
I also think rubbing the ears and washing the face can happen here and there, disconnected from an exercise set. I often do shorter versions of harvesting qi after meditation.
Margaret,,,,"Shawn" is the LS moderator in this section.