A tablespoon is a non-scientific but very widely used unit of volume, which works very well in cooking, herbal medicine, etc. It is represented by a capacity of one tablespoon which varies from roughly 15 mL in the US to 20 mL in Australia.
1 If tea is properly stored, it can potentially last up to a year for green and black teas. White tea lasts about six months before it isn't as good in quality as when fresher. Stale tea (or bad tea) will taste flat, as if you're drinking paper.  Glass or ceramic containers are fine provided that they're opaque.
Loose green tea is a perishable product. The rule of thumb is that green tea turns hay after 6 months. If you want to keep it fresh - keep it in the cooler (4-5C/40F) in zip-locked package. Green tea easily absorbs any flavors, so if you don't want your lovely Bi Luo Chun smell like salami, keep it zipped. The only exception to 6 months expiration is vacuum. If you vacuum your green tea and put it in the fridge you can prolong its life for as long as 2-3 years. All of the above is true for good quality tea carefully produced by responsible farmers.