While vanilla beans won't get rancid, they will lose their flavor if not stored properly. Kept in an airtight container, away from heat and moisture, they should last almost indefinietely.

You can use older beans for vanilla sugar, or even beans that have already been used in another recipe (rince used beans to clean them and let them dry). To make the flavored sugar, tuck whole or cut-up beans into an airtight container of sugar, using 2 cups of sugar for each bean. Let the mixture sit for at least a couple of weeks before using.

As you use up the sugar, you can add fresh sugar and more beans if you'd like, but you don't need to replace a bean until it's lost its scent.