How Soon is Too Soon to Sign Your SO’s Name to Christmas Cards?

Finishing my Christmas cards is among the many things on my list of "to-dos" before leaving for vacation.  At one point, I was faced with a very difficult question...

My boyfriend and I are both from Ohio, so neither of us shared Thanksgiving dinner with family this year.  Instead, we had two other couples join us at his place for a small "friendsgiving" dinner.  I was writing a card for one of the couples and wrote, "Thank you so much for joining us for Thanksgiving this year! Have a Merry Christmas and a Happy New Year!" But when it came to signing it, it felt really weird signing just my name when I said thanks for joining "us."  Ultimately I ended up just signing my own name instead of both mine and my boyfriend's.  We've been dating less than a year and it just felt "too soon."

I didn't want to be "that girlfriend" that went ahead and signed both of our names without asking him.  One of my former co-workers had a girlfriend that did that to him without his knowledge.  He didn't know she'd sent cards from the both of them until one of our other co-workers thanked him for the card from him and his significant other.

Is there a specific amount of time you should be dating before signing your significant other's name to your Christmas cards?