VNC Timer

Hello fellows,
I am new in this forum, i would appreciate your assistance.

I need a timming system for my vnc desktops (Cybercafe timer stuff).
Each unix user login to my server only with vnc, and i want to write a program that can generate timer tickets and have control on the time used for the session.
I am thinking of writing some shell scripts along with cron jobs, then later build a webbased user interface using perl or php.
is this that the best way to go about this?
I would appreciate some tips from you.
Thanx