Comment 6 for bug 217936

>That's handy to know. Although it seems to me that internet tariffs with "unlimited" time and "pay-per-GB" time are reasonably common, so it would be useful if update-manager had a GUI option to set a time at which to start the process. Or even a way to say "Run an update at time X:XX every Y days."

Try the package: gnome-schedule
For system commands: gksu gnome-schedule

http://gnome-schedule.sourceforge.net/

It uses /etc/cronrab (or similar files) to create the rules and run them.
You could set a rule to run a script (name the following and save it as e.g. /home/update.sh ):

#!/bin/bash
aptitude update
aptitude -y upgrade