Thermage tightens the collagen of the skin in the treated area, which tightens the skin. You will see the results gradually over 1 to 6 months after treatment. The changes don't "go away" but the aging process continues, so the collagen that was tightened will eventually loosen again. How often you get treated depends on your age, skin condition, and what your goals are. You can do it as often as every 6 months or every couple of years, to keep your skin tightened and postpone the need for surgery for a very long time.
Almost everyone sees improvement with Thermage when it is done properly. The results range from the subtle to the dramatic. You will see the full result in 4 months, and after that it is long lastng.