It lasts for better or for worse, until death.
You will get xp from group kills and share quest updates quite some distance from others as in closer to 100 than 28.You have to be in the range where you share XP. If you look at your group bar, if the other person's bar gets darker they are too far.
I think of it as "healing range", since I'm usually the healer. If you are out of healing range, you are dead to me.