I have been using the LFD tool to queue for Heroic Dungeons.
First I noticed that you do not get a 30min “deserter” debuf if you leave past the first or 2nd boss. Also there is no 15 min timer which I think exists for M+ ?
But eventually I got a message “you or someone in your party has entered too many instances”.
It is not for M+. If the group spend some time after the first/second boss, if some people (3 people I think but not sure) leave the instance before you, the AI perceives it the group is not qualified to finish the run and doesn’t penalise the leavers.
Not sure again but it should be if you are grouped with someone else that is somehow restricted to queueing for the dungeon, you get that warning or it is a bug.
A player can only enter ten individual instances in one hour. This even applies to the same instance when a player uses the Instance reset ten times within an hour, because each reset is considered by the game to be a different instance.