For Life was private with a very specific set of rules. It was a niche' world and, when asked to join, I told them as much. I recall playing a season there and really feeling like there wasn't much to do in way of making your team better from season to season. It was definitely a long haul world. The inability to fill should not be surprising.
As for how long I'd wait, it depends. If I had 2-3 worlds playing, waiting is no big deal. If all my worlds are idle, I'd want to play somewhere. As someone mentioned a few days ago, the potential biggest problem WifS has is owners realizing they don't miss playing. There is a "routine" involved and, most likely, all the long-term owners have fallen into one. If I had one world with 12 openings, I'd be less inclined to check in with any regularity. Suddenly, I'm not checking in at all.