Carbon loses most of its eficacy in just a few days. Don't replace all your carbon at once though because it can hold a significant amount of denitrifying bacteria in your tank, which is helping you with filtration. Just swap half at a time, so that your tank can adjust accordingly.
I swap half of my carbon every week when I do a water change. It's easiest to just use 2 bags, each containing half the amount you would normally use. Just pull a bag out and swap it for a new bag and you can rotate between new and old carbon.