With a syrinx I have known this to happen in a very short time frame...the syrinx can grow rapidly and symptoms progress just as fast...or it can go very slowly....the problem with the slower progression is u adjust to the symptoms as they occur and u do not notice them, but they r just as harmful as those that might seem to appear over night.
Compression of the spinal cord is nothing to mess with...and I understand ur concerns, and y I would suggest an opinion of a diff Dr to see what they say...just go to one that is a specialist in the area of Chiari and syringomyelia.
Make sure they rule out all related issues first...no EDS, not scar tissue...or cerebral ptosis b4 u consider a shunt....as it could be a matter or treating the other issues and no shunt may be needed.
As far as if it is a large syrinx, I will say the same thing I say regarding a herniation of the cerebral tonsils...it is not how long, but how wide it is, as that is when the damage is being done.,....in ur DD's case where it is will depend on how she is affected as to which nerves r in that area....I have a chart that shows what could be affect in those areas-
http://www.medhelp.org/user_photos/show/197978?personal_page_id=197028&photo_collection_id=1369