Comment 33 for bug 53868

I've found some good information from bug #369198. Especially comment #10.

[QUOTE Matthew Paul Thomas from bug #369198, comment #10]
When non-security updates are available, Update Manager should open automatically whichever is later: (a) X days after it was last open, whether that was manually or automatically, or (b) X days after updates were installed by any apt-powered method. X should default to 7.

When security updates are available, Update Manager should open automatically whichever is later: (a) Y days after it was last open, whether that was manually or automatically, or (b) Y days after updates were installed by any apt-powered method. Y should default to 1.

So yes, it is intentional that running apt-get update shouldn't cause Update Manager to open automatically. If you run apt-get update manually, it's highly likely that you're about to install updates manually too, in which case you don't want Update Manager getting in your way.
[END QUOTE]

I generally end up updating manually before 7 days that's why it didn't show up on me. So I kind of brought this bug back to life for something which isn't a bug but a feature... (I'm sorry for that :-))

Anyway, if someone reads this and wants to change it search google for '/apps/update-notifier/regular_auto_launch_interval'. I'm leaving it as default for now.