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.
Was hoping for longer, though! I make and consume iced tea all summer long and try to make the amount we will consume in about 3 or 4 days. Beyond that time it starts to taste stale. If you keep the tea refrigerated the 3 - 4 days sounds right. I have had iced tea served to me in restaurants that has the off taste of mold. I assume that means it's not kept in the fridge and/or kept too long.