Certainly you can conduct sorptivity testing for any time period in which you are interested. In mortars and concretes, longer term sorption may be related to the infilling of air voids by solution that can have major implications for freeze-thaw durability, for example.

Sorptivity tests basically provides information about the transmission of water into the material through capillary suction. The penetration of moisture through this process may give the indirect information about the micro-structure of the material which resists the ingress of hostile products and affect the durability of concrete. Long term tests are desirable to ensure the capacity of material to prevent ingress of water by all means including Sorptivity.

Yes, you can perform it on long term curing condition as well. I myself perform it after 90 days curing time.