This entire issue was recently, lengthily addressed in our parish bulletin, with the priests going so far as to say that those who leave early (after communion or before the dismissal, as well as even arriving late) have not attended a valid Mass. I have another pet peeve -- which is people who bring "baggage" to communion (large purses, etc.). Sometimes they do this because they have every intention of making an immediate exit; other times because they can't seem to part with their baggage for some emotional reason. But I won't hijack the thread regarding that, just wanted to add that as an indicator.
It's to be noted also that this is a solidly (even retro) parish, with the vast majority specifically seeking it out because of its traditional practices, as well as OF and EF Masses (even daily Latin Mass), so it's surprising to me that this even has to be addressed, but apparently there are those who still do not see the liturgy as a unified whole and do not see themselves as a participant in a unified Mystical Body. Sad.
The priests enumerated what can be classified as an emergency, justifying early departure. I don't know how many leave early in this case, because I rarely see it at this parish, and I usually sit toward the front in any case. But it would have to be of some significance (or some constancy with at least a few parishioners), or no mention would have been made of it in the bulletin.
In other parishes, it's an all-out epidemic. There have people -- esp. women -- who come quite late, chew gum and eat mints during Mass (ceasing just barely prior to communion), bring everything and the kitchen sink with them into church, then bring all their accessories with them as well to communion, and leave immediately afterward. That's one of the many reasons I never go to a typical parish unless I absolutely have no choice.