Sorry. Misread.
There are a multitude of third party applications for doing this.
However, if you want to log off at a specific time or after a static number of minutes you can easily accomplish this with task scheduler.
The command shutdown /l does not appear to support the /t parameter, so when you set up a task, in the task properties, you can set it to "delay" for a specified period (Eg. 30 minutes).
You could create a shortcut on your desktop to run the task, so as soon as you run it, the computer will log off after that many minutes.
Or you could set the task to run at a fixed time and have it log off immediately at the specified time.
Is that what you are after?