How often do you get rid of your older movies because they no longer sell?

I was just wondering what the shelf-life of a DVD/VHS movie was in terms of customers actually buying them, or do they keep getting bought forever?

Id imagine, based on what others are saying about surfers looking for new content on websites, that once you sell a certain amount of copies of your movie titles you just throw them away because they are 'old' and customers no longer want to buy them.

So how often do you discard your older movie titles once they stop selling or do they continue sell for many years after their initial date of production?

Regards,

Lee