Unlike loose leaf tea, which won't fit in a bag, the tea in tea bags consists of chopped leaves and dusty particles called fannings. These smaller pieces are more prone to oxidation and turning stale. Your best bet is to buy tea bags in quantities that you think you can use up within six months, so that the flavor and aroma remain strong, and before it becomes a bit more stale after one year. Add a note to the container holding the stored teabags when you bring them home, so you know how long to keep them.
Try to finish delicately flavored teas such as Darjeeling within two to three months.
A storage canister can slow oxidation and allow tea bags a longer shelf life. To keep moisture, oxygen and rival smells away from your tea bags, place them in an airtight, preferably metal, container and away from sunlight, heat and high humidity. This may keep the tea fresh for up to two years.
An exception to this guideline pertains to the aged Keemun and Pu-erh teas, which like wine require oxygen -- but not heat and moisture -- to continue to mature. A fermented tea made from a leaf grown in Yunnan in China, pur-erh tea can stay fresh for up to 50 years.
Tea purveyors came up with the six-month guideline for best results, but if you aren't picky and don't want to waste tea bags, you may be able to store them for up to two years. A doctoral dissertation filed with the Department of Human Nutrition of Kansas State University, for example, studied green tea at intervals from three months to two years after the original packing dates. Taste testers found minimal changes after one year and only slightly greater change during the second year of storage.
Further, teas never spoil, so you can experiment with older tea bags -- particularly black leaf -- to see if it still has flavor even after a year or two. If they do make poorly flavored tea, add the tea bags to your compost heap.