Until someone more senior than you comes along and takes your seat. Typically someone from outside the office who just wants to sit next to their "friends". Nothing quite reminds you how junior you are than to have to find another desk.

It's another one of those "wins" for the corporation - why set up guest stations throughout the business when you can just say "there are no assigned desks"? It even re-inforces the "we are a family, not a company" meme.