To lock the screen, you can follow the answer to this post.
So you would write a script (in GEdit) containing:
#: filename: lock-script
and save it in your home folder (or you could create a new directory for it).
If you only one to black out your display (without locking) change the
-l switch for
Make your script executable running
chmod 744 /home/your-username/lock-script.
Then execute the following
echo '15 * * * * /home/your-username/lock-script' | crontab
lock-script will be executed 15 minutes after the start of every hour on every day. (At
01:15 pm, then at
02:15 pm, and so on.)
If you want more information about how to specify the period of time, you should check here.
BTW, for creating the
crontab line you could install
gnome-schedule which is a GUI frontend for this.
Hope it helps you!