Thats precisely why I said now's the best time to moan how crappy Steam can be!
To prevent HL2 from being pirated, Valve came up with Steam. Now when you install the game demo or full version, the game files are in an encrypted form and not ready for play. Steam will connect to the Steam servers and decrypt the game files so that the game would be ready for offline play.
The time being displayed is the time that it will take to decrypt all the files and make HL2 availble for offline play. I guess the Steam servers are too busy today. Generally people with dial-up have been able to decrypt the
full-version game files in around 4 hrs. Thats what most of them whom I talked with have told me. Wonder why your demo is showing so much of time.