Some of my collegues in Los Angeles tell me there are celebs doing it every 3 months (I am not recommending this).  We really don't have any studies to answer the question either.  Since it's a sound wave, and sound waves have been used for various purposes in medicine for several decades, Thermage is considered very safe. 

Everyone gets a increase in collagen from it in the dermis (deeper layer of skin)  - even those who don't see a visible result.  There are good studies showing this.  I usually tell my patients once or twice a year partly because because there is no evidence that doing it more often is helpful.  And, it's expensive. Every six months should be fine.  Dr. I