If you look for information about the amount of caffeine present in a cup of tea, the amounts vary depending on the kind of tea, if the tea is bagged or loose leaves, the brand, and other factors. Nonetheless, the values estimated range from the 25 mg in a cup of white tea to 50 mg in a cup of black tea, or maybe more.
Let's say I use 2 grams of black tea leaves and I brew them in 200 ml of almost-boiling water for the reccomended time of 5 minutes. I get a cup of tea with an estimated amount of caffeine of 50 mg. But then I reuse those same leaves and I brew them again in another 200 ml of water.
What would be the estimated amount of caffeine in that second brewing? Are there estimates for that? Can the result be at least expressed in an estimated percentage with respect to the amount obtained in the first brewing? Would that percentage be the same for other kinds of tea (green, red, oolong...)?