Over a year ago I made flavored sugars and have recently remembered them. They are basically a sealed container (canning jar) with a cinnamon stick, lemon/orange zest, vanilla bean/pod, or a combination of two of the aforementioned.
They're good pretty much forever. There is not enough available water for things to grow in there. Mind you, the more aromatic and subtle parts of the flavor will dissipate over time.
The sugar absorbs water, giving the combination a very low water activity, meaning there's little water available for microorganisms to use in growth. As long as your flavoring pieces are small and well mixed with sugar they will not spoil.