The time you want until the item is not available anymore. The fact that you have it in your cart doesn't mean that you have it reserved for yourself... You actually have to pay for it to have one copy ordered for yourself.

What I mean is that if you plan getting the LE DVD version or the first press of the cd, you should buy both as soon as possible (paying them). That you have them in your cart, doesn't count. If other people keep buying them and they go out of stock, although you have them in your cart, you won't have them.

Hope I explained myself :S ;)