I have not started really hanging yet. But when I do, I have to agree with SWM that 20 min sessions a few times a day is best. I base this on medical info I picked up in a burn unit I worked in.
When the physical therapists come in to stretch a patient's ligaments and skin so that they do not retract, they do so several times a day around the clock. Every 6 hours at least I have heard is best between stretches in order to prevent the body from retracting what you have just stretched. By doing so in this manner, consistent and permanent gains can be made with the tough tissue of ligs .
When I get my hanger, I think I will start a routine that will be stretching 3-4 times a day as evenly spaced out as I can with my schedule. I think SWM is right on track with that idea. And the hanging weight I will use will be one which will not let the ligs "accomodate" too quickly to the weight and which would just let the ligs and tunica get tougher and stronger without the stretch that I want. So it needs to be fairly heavy but not heavy enough to cause damage and hurt. You need to stay "ahead of the game" when it comes to accomodation and your body wanting to always return to normal.