If you are running a copy of Vista that is not activated, you can see how much time is remaining by using this command:

slmgr -dli

To see the actual date and time that Vista will expire, run this command:

slmgr -xpr

You can reset that timer by running this command (as admin if you're using UAC) and rebooting:

slmgr -rearm

According to Microsoft's website, that rearm command will only work three times. So basically, you can extend your trial period up to 120 days.