Both my kids wore them full time from 3 months old until they asked to take them off (both at around age 2.5 when they had all their teeth). I don't know if they work, but neither of my kids ever got the red cheeks, I was never up with them teething, I never used any panadol or any other pain relief. I rarely knew when they were getting a tooth. Which seemed in contrast to my friends who used to say their baby was up screaming all night with teething.
In the photo I saw of that girl with her arm through it the necklace looked way too long for her.