This makes me consider the opposite - that I really don't think any date before Christmas is "too late". Since I plan long and hard for finding meaningful cards, and try to make it a family experience for dd, I would like to think they arrive at friend's homes early enough to be a part of any decorating they may do. And that they don't arrive so late as to maybe be lost in the shuffle of that household's frantic moments. But if I got the cards into my friends homes on Christmas Eve, I would still call that a "win"!
Here's another thought or memory jogger - is there anyone on your list to whom you also send a birthday card during the season? HRH's brother's bday is right in the middle of Christmas Card arrivals. I try to make his birthday card separate and special, and send their Christmas card at least a few days apart from it. This is a time when I will definitely use a "Happy Birthday" seal/sticker and won't use a Christmas stamp or Christmas return address label, because I want him to see his birthday is as unique a day to us as anyone else's through the year.