While Steam theoretically allows you to play your games without an internet connection, there are catches, like the fact that every now and then you have to get back online for the feature to work. Which in some circumstances kinda defeats the point!
Weirdly, while you'd be forgiven for thinking this was a design decision, turns out it's just a bug. One that Valve is trying to fix.
"Offline Mode is designed to be indefinite", writes Valve employee Henry Goffin on the company's forums. "You can't access any of Steam's online features such as friends lists or saved game synchronization, of course, but the client should allow you to run in Offline Mode for as long as you like."
"That said, there are many components involved in Offline Mode, and some of them have known issues and bugs which we are continually working to improve. We're aware that it doesn't always work as flawlessly as we want it to, but please keep reporting bugs with Offline Mode. It is not broken 'by design'."
Well there you go. No wonder nobody could ever get a definitive timeframe or workaround for the thing.