New fb chat is kind of confusing. How long do i have to not do anything to be idle? Help?

[miniaturehorse036]

EDIT: Actually, I think the new facebook chat does not feature the idle symbol anymore.
There's only a green dot for availablity, and none if they are unavailable. It's unclear whether being idle for a set period of time (somewhere between 10~15 minutes) is considered being unavailable.
Idling seemed to have been taken out of the Facebook Help Centre, so there's no documentation on the feature anymore.

The new FB chat updates based on various unknown factors. Some of them disappear once they are unavailable, regardless of whether they are actually offline or just being invisible. It updates based on recency, amount of communication between you and them, and much more. You can search for any person at any time by typing their name into the search box on the chat window.