The minimum time I could give to a user is 1 day ! This is made through the Principal Expiration and Password expiration at the Kerberos 5 menu of USERS, by giving the date of the next day!
How can I give less time to a user?
How does ticket life affect the expiration of connection time?
At the moment the K5 Ticket Life Time does not affect the captive portal authentication validity. This feature will be included with the accounting module that allows to limit the connection time and the amount of traffic.