If using the internal captive portal you can force the users to re-authenticate periodically by setting the "Reauth Interval". By default this is set to "0" minutes meaning no reauth. Go to your guest SSID and select "Edit" then click next till you get to the Security screen then set the desired reauth interval (in munutes or hrs).
Reauth interval— When set to a value greater than zero, the Access Points periodically reauthenticate all associated and authenticated clients.
