How to keep Steam in Offline Mode?

You can stay offline indefinitely. (I know it works -- I have been doing it for many years now.)

You will need a dedicated PC (or, possibly a virtual machine?) that never connects to the internet -- at least not while Steam is running (either foreground or background).

Now, Windows 7 has been my dedicated machine for this. And you may need to take action when you get the little "Create new account / Login to an existing account" dialog -- which I can confirm happens to me occasionally, maybe even every 2-3 weeks.

Here is what you can do:

• shutdown Steam completely
• add the following entries to your Steam settings (in the version I'm using, they are in the Windows Registry...ugh... hopefully this is different in Ubuntu)

The AutoLoginUser and RememberPassword are the critical pieces of data here.
You should not be prompted to enter your password again, provided it was entered correctly while the computer was online when setting this up at the beginning.

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Valve]

[HKEY_CURRENT_USER\Software\Valve\Steam]
"SuppressAutoRun"=dword:00000000
"Restart"=dword:00000000
"Language"="english"
"BigPictureInForeground"=dword:00000000
"AutoLoginUser"="<YOUR STEAM USER NAME>"
"RememberPassword"=dword:00000001
"TempAppCmdLine"=""
"TempAppPath"=""
"AlreadyRetriedOfflineMode"=dword:00000000
"WebHelperFirewall"=dword:00000001

If you want to earn Steam achievements, you can do that too!
Just find the correct file, usually looks like:
<STEAM ROOT>/appcache/stats/UserGameStats_<YOUR STEAM ID>_<STEAM STORE GAME ID>.bin
or you can search for recently modified files.

That file can be copied to a Steam installation on another computer with your account, start Steam in online mode, and launch the game. All earned achievements will be triggered at the moment the game starts.