I am running ubuntu 11.10. in my version the automount command (aka the running daemon) has two ways:
1) if the timeout is set to zero, then they will never unmount. in ubuntu, this value is set in /etc/defaults/autofs.
2) sending a USR1 signal to the running automount daemon will unmount all unused filesystem and keep running. (forced expire)
3) sending a TERM signal to the running automount daemon will unmount all unused filesystems and if there are no more, shutdown.
Bear in mind, if you have an unused shell prompt whose current working directory (cwd) is in one of those auto-mounted filesystems, it will not be unmounted: the file system will be "busy".