As the server doesn't know whether you're still active or not, it assumes a 10 min timout. If you were active in the last 10 min, other people in the same room cam “see” you. Similarly, if you “say” something, whatever you said hangs on the room description for up to 10min as long as your character is still in the room.

One planned feature is that the world should shrink if it isn't being used. Thus, there's no point in creating a vast number of rooms with long descriptions, unless you're visiting them on a regular basis.