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.
All teas benefit from being stored in a cool dry place where the temperature does not fluctuate very much. Tea is very good at absorbing odours so keep it away from anything smelly. Airtight containers are inexpensive and much better than the plastic pouches or cardboard containers that tea typically comes in. Almost all teas last for about one year.