Activated carbon in a dry environment can last indefinitely. But if wet the the large surface area that carbon has promotes algae and mold growth. Once this happens the treatment is almost expensive as the cure (chlorination with hypo which tends to burn the carbon). If taking a column off line for an extended period of time make sure the unit is drained and then I would purge the system with clean dry air for approx 24 hrs. If you then flange or valve off both ends you should be OK for a extended period of time. Corrosion of the vessel itself would be your next concern.
