There should be both a graphical and a config file way to do this. The graphical way probably depends on the desktop environment. If you are using Gnome it probably works similar to what I have in Ubuntu:
System -> Administration -> Software Sources
has a tab called Internet Updates
where you can enable/disable it and select a period for updates. (Yeah, that's not terribly intuitive!)
If you prefer config files, I have a file called /etc/apt/apt.conf.d/10periodic
with a line
This shows automatic update disabled. Changing the "0" to a "2", for example, would check for updates every second day ("1" for every day, etc.).
If you want to look under the hood and see how all of this works, there is (on my system) a script called /etc/crond.daily/apt
which handles this. Along with everything else in this directory it should run once a day (regardless of your configuration).
Note: The leading two digits on the 10periodic
file might be different in your distro. Don't worry about such a difference.