Valve employee henryg responded to accusations in the Steam forums that the Offline mode had been programmed to fail every two weeks. He stated:
This is not actually true – Offline Mode is designed to be indefinite. 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.”
He then further clarified in a later post on why inquiries about this and other issues take so long to be answered:
We have been working very hard on upgrading the underlying technology in Steam. We have no community managers or PR people, so all the time that we spend on forums is taking away from development time. That said, I know that it’s frustrating to get no response, and it looks like we aren’t doing anything about it.
Hopefully the Offline mode issue will be resolved soon.