How fast can a syrinx progress?

variable: This ultimately depends on what's causing the syrinx. The most common cause of syrinx is post-traumatic, and these tend not to change. However, those caused by structural anomalies or tumors can enlarge, probably on the order of months to years.

Depends on the cause: Your neurosurgeon can decide, based on your neurological history and exam, and MRI scan, whether the syrinx is clinically relevant, needs follow up or surgical treatment. The surgeon can also explain what may have caused the syrinx, and predict the likelihood of progression.