How long the treatment should last?

It depends on which treatment style I am using and to some extent what the persons condition is. And this will be the same answer for most practitioners - it depends on the system they use and the patients condition. 25-40 minutes of having the needles in is probably the range - possibly longer for tonifying deficient patients and possibly shorter time for removing excess. Also, it seems to me that people have cycles of relaxation - they will get deeply relaxed, wake a bit, relax again, wake, etc. The first cycle seems to be the strongest and lasts somewhere in the range of 20 - 30 minutes range, so longer treatments seems unnecessary. If the treatment is longer it is usually from switching from front to back needling - and/or adding moxa, tuina, etc..

The vast majority of the time I am using the Tam Healing System for treatment, so here we do a few minutes of tuina first, needle and leave them in for about 25-30 minutes, and then do about 10 minutes of tuina along the spine. The Japanese Acupuncture section has all of the timings for each stage - the protocols involve needling the front, back, moxa, etc. so the timing is fairly detailed.