According to In Pursuit of Tea, tea stays fresh for up to one year when stored in the right conditions. Optimal conditions for tea storage are vacuum-packed or nitrogen-flushed in moisture- and temperature-controlled environments.
Every type of tea is different, and a several variables affect storage conditions. Certain green teas are fragile and lose finer notes quicker, while other teas are easily stored for a few years. Less oxidized leaves lose their flavor faster once exposed to oxygen. Darker oolongs and black teas tend to keep their flavor for longer periods of time. Some of these teas can last for more than two years.