How long does tea retain its taste?

Unused tea bags and loose tea last for 1-2 years on the shelf, depending on the type of tea, when stored properly; green tea and white tea have a shorter shelf life (some of our sources go as low as within 6 months of purchase), while black tea and oolong tea have the greatest endurance.
 
The extensive list of expiration dates provided by the Texas Agricultural Extension Service boils it down even more concisely: Keep tea bags a maximum of 18 months, instant tea 3 years, and loose tea leaves 2 years. If you’re an infrequent tea user and expect to keep it a long time, write the date of purchase on the product.
 
Source(s):
Texas Agricultural Extension Service: "Safe Home Food Storage"
E-How: “How Long Do Tea Bags Stay Fresh?”