Keep in mind that tea is vulnerable to 5 things: air, light, odor, heat, and moisture. When tea is excessively exposed to these 5 elements, it will gradually lose flavor and become stale.
If you plan on not using your tea for a while, the best place to store it would be in double-lidded, airtight tin canisters or ceramic containers. Another very important rule to remember is to always keep tea at cool and dry places.
In general, oolong and black tea could be kept between 2-3 years under ideal conditions. Tea that is less oxidized has a shorter shelf time. Green and white tea could remain fresh for up to 2 years if stored properly.