We very rarely get cards these days. It was my parents' generation that sent them, and there is only one person of that generation left now, and she has long since embraced e-mails.
I never have displayed birthday cards after I grew up. On birthdays, graduation and other celebrations we placed the cards that did not contain very private messages on the table along side the gifts received. There visitors were welcome to look at them.
Unless a card meant something very special to me, I haven't every kept them once the celebration was over. I realised in my early twenties that there was no point in having a shoe-box or a drawer full of mementos I rarely, if ever, look at.
Once or twice a card escaped being thrown out, but on two occasions I found such a card after the sender had died, which I found acutely distressing, so since then, I have been meticulous about not keeping letters or cards.
I have never heard there is a length of time for keeping or displaying this sort of thing, I imagine everyone does what they want about it.
