The percentage of ingredients in a glass of tea
You can't really talk about the 'concentration' of tea, because 'tea' is not a well defined compound. It contains a lot of different chemicals, some of which are dissolved into water when brewing.

You could talk about the concentration of caffeine, epigallocatechin gallate, or manganese in a particular cup of tea, but they depend on the type of tea, the temperature of the water, the infusion time, etc.

The minimum strength of a cup of tea that still allows it to be called tea is a fuzzy question without an answer. Take a pile of sand. Remove grains of sand one by one. When does it stop being a pile? A reasonable limit for the tea would be when a normal human being can't taste the tea anymore.