When we teach our retreats people can change as often as they want. We encourage people to stay still as long as they can, but as soon as they get too much pain or discomfort, it's quite OK, shift the legs, relax the back, put one leg out, put out two. The average person sits most of the time bent over. Whether it's on the floor or in a lounge chair or whatever, they are slouched. So their lower back muscles are usually weak. The sitting meditation posture is for a large part is like a physical sport, we have to strengthen these lower back muscles, stretch out other tight muscles,. So we teach people to shift their posture whenever they need to. During retreats we give stretching exercises with mindfulness to help strengthen the posture and we teach techniques on how to work with body pain. It's amazing how many people come into their interview following that talk and say: "I sat still the whole 30 minutes this morning, first time, oh it's so wonderful, that talk last night was so good!" and they are bubbling, because of these extra techniques we give people to work with body pain. However, again, we say change your posture as often as you need to, but as you practice you may find that you will be able to sit still and more peaceful for a longer period of time.