Like most protections in LFR, it depends on player, the player's accumulated infractions, the player's number of successful runs, and the number of attempts made at a boss. If you're tagging along with a friend that has to bounce after a kill or two, you're likely to be fine. If you're buying an instant queue in trade, chances are that you're actually paying for a deserter debuff.
Recently, I joined an...infamous tank for a queue and I still got the boot after the Durumu kill when he dropped, along with the three other players in our group.