Dear mcgregox,
Zeroshell does not support time limit connection with ticket. In the future, as Fulvio said, there will be a version of zeroshell with accounting. By that time time limit will be posible.
I am also waiting for this accounting module of Zeroshell more than a year now!
I am not compaining because Fulvio have given us a wonderful piece of software that is doing a lot of work the way it is now!