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.
If it is black, it could last years, 2 to 3. If it is green, it may last only months. Once a green tea is opened, it may only be good for a few months. But I have had some greens open for 6 months and they actually were better, but this is the exception. Same with oolong. Let your palate be your guide.