Caffeine in Tea varies quite a bit. The longer the tea (or tea bag) is steeped in water, the more caffeine will be present. On average loose black tea had 22-28mg of caffeine per 1 gram dried matter.
The amount listed above is an average amount taken from various laboratory tests. Originally we listed an amount (47mg/8 fl oz brew) taken from the USDA. However more recent lab tests show this amount to be lower.