These areas are different from other parts of the body. On average 15% of the hairs are in the active phase. This is less than other parts of the body, which is why generally two more laser treatments are necessary.

Hair on back and shoulders is thin to medium thickness, and the density can vary enormously. These factors affect the number of laser treatments you will need.
Obviously, you can stop the treatment whenever you find the result acceptable. This is a highly personal matter. In any event, you will experience a significant reduction of hair growth on this part of your body.