Hi
The default log off period the switch waits for
client activity before removing an inactive client from the port is 5 minutes (300 sec), and that can be set by the option: aaa port-access authenticator < port-list > [logoff-period]< 1 - 999999999 >
One more thing, the re-auth period which is the time after which clients connected must be re-authenticated is by default 0 sec (means disabled), so you can try enable it by the option:
aaa port-access authenticator < port-list > [reauth-period < 0 - 9999999 >]
Also, if you are trying to do that with one port on the switch, then you have a Limitation on Using an Unauthorized-Client VLAN on an 802.1X Port Configured to Allow Multiple-Client Access.
A client using the Unauthenticated-Client VLAN will be blocked when another client becomes authenticated on the port.
For this reason, the best utilization of the Unauthorized-Client VLAN feature is in instances where only one client is allowed per-port.
Good Luck !!!