How long?

How long should you keep cards up on the mantelpiece for? They cost so much to buy you feel you want to show them off for a decent period of time but how long is 'decent period of time?'
Regardless I am taking them down tonight because the dust is gathering around them and it's awkward dusting round them. Also there comes a point where you think they are just making everything look untidy! So into the scrapbook they are going!