Dead Letter Etiquette

What is the requisite length of time you should keep a card? . . . birthday, Christmas, or otherwise . . . and does it matter who gave you the card? does it matter that you're not going to read it again? does it matter if it has a picture of someone's family on it? . . . I believe that this story makes a case for keeping personal correspondence for a great length of time and for destroying it immediately . . . but I feel like I stash holiday cards in a basket for what I consider to be some arbitrarily polite length of time, and then I finally toss them, because some part of me feels like it would be rude to just read them, look at the picture, and then immediately chuck them in the trash . . . stupid card industry causing me more grief.