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.
Black teas however rely on complex polyphenols for flavor, which atrophy at a much slower rategood for at least year, sometimes two, three or more. As a general rule, the larger the leaf and more tightly rolled the tea (e.g. pearls, oolongs), the longer it will stay fresh.