What is the hallmark?

I wish there was a written expiration period on "greeting" cards so I didn't feel like a butt when I tossed them. What is the standard? Do you keep "thank you" cards forever but toss a "get well" when you get well? Do you cherish a card with hand-written, heart-felt words added but chuck one when the sender only underlined a word or two? Why can't you read it and toss it immediately or just give it right back and say ditto? I like cards mind you. I'm just not sure about the post-sentiment�etiquette.