It is very difficult to know the exact amount of caffeine that anyone consumes. You can buy tea bags that come in regular, strong and extra strong, all with differing caffeine levels. Then you might be a jiggler or dangler, all influencing your caffeine intake. Be aware that green tea has the same amount of caffeine as regular tea. A strong cup of coffee will provide about 80mg caffeine, about the same as an “energy” drink. A five minute brew of tea is around 30-50mg caffeine. Two teaspoons of Milo or drinking chocolate has about 2mg caffeine. A can of regular or diet cola will have about 50mg caffeine. Dark chocolate will have around 60mg of caffeine in a 100g block.
If caffeine alone is proven to slow cognitive decline, then the above research hints that somewhere in the region of 150-300mg caffeine is a healthy “dose”.