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.
Pepsi invited me to come see Pure Leaf's brewing process at a plant in Indianapolis, where workers dunked 400 pounds of tea leaves into an industrial kettle. Each batch produces 40,000 to 45,000 18j.5 ounce bottles of tea. The trick is to harvest and blend teas from around the world and keeping the taste consistent.