Code:kNoRepeatNetworkAlertSeconds : 15, kNetDownForceQuitAfterMinutes : 20,
So if the game detects you are offline it starts a 20 minute countdown, if you are not online before then it will force you to quit.
For those who are interested, extracted/abridged source code of the always-online: http://pastebin.com/8NR1EdkN
Simply commenting out line# 22 defeats the force disconnect.
Removing this line allows indefinite offline play.
Best part being that the .package format they're using for Simcity looks to be the exact same as the one used for The Sims 3, encryption and all. Do I need to tell you that the Sims 3 .package format has been completely hacked and it's possible to both open and create .package files to use for modding in that game? Anything that's in a .package file might as well be stored as plain text.
They could at least have changed the format if they didn't want people to open up the code on day one... It took several months going from Sims 2 to Sims 3 modding since the .package format did change between them. But, you know, they didn't.