From the description given in the testimony, the fiber was wrapped around hair that was tied tightly and wrapped around the necklace. (I may be misqouting, this is what I remember) Sounds like the fiber could have been there ever since she started wearing the choker. It could have gotten their when she was abducted/died. It could have gotten there 1 week or 2 weeks, or 6 weeks, or 120 weeks before that.
All of these are just as possible.
Remember, the hair with the fiber was "STUCK" in the choker, and Danielle NEVER took the CHOKER off for TWO YEARS.
I would agree with that only if the hair was not also attached to her head. If that was the case, I would have to think that the hair got entangled during her abduction and her possibly fighting off the perpetrator. Otherwise, as you stated, that hair could have been there as long as she wore the necklace. I remember finding bits of my hair entangled in a gold chain I wore and rarely took off.