A reliable source for the caffeine content in tea

There is an ISO standard for tea preparation.

To quote the summary:

• The pot should be white porcelain or glazed earthenware and have a partly serrated edge. It should have a lid that fits loosely inside the pot.
• If a large pot is used, it should hold a maximum of 310 ml (±8 ml) and must weigh 200 g (±10 g).
• If a small pot is used, it should hold a maximum of 150 ml (±4 ml) and must weigh 118 g (±10 g).
• 2 grams of tea (measured to ±2% accuracy) per 100 ml boiling water is placed into the pot.
• Freshly boiling water is poured into the pot to within 4-6 mm of the brim. Allow 20 seconds for water to cool.
• The water should be similar to the drinking water where the tea will be consumed
• Brewing time is six minutes.
• The brewed tea is then poured into a white porcelain or glazed earthenware bowl.
• If a large bowl is used, it must have a capacity of 380 ml and weigh 200 g (±20 g)
• If a small bowl is used, it must have a capacity of 200 ml and weigh 105 g (±20 g)
• If the test involves milk, then it can be added before or after pouring the infused tea.
• Milk added after the pouring of tea is best tasted when the liquid is between 65 - 80 °C.
• 5 ml of milk for the large bowl, or 2.5 ml for the small bowl, is used.

First off, the ISO standard is not intended to produce a good cup of tea. It is designed to produce a consistent one for taste testing, so that no tea manufacturer can claim that his tea wasn't made "properly". It's title is "Tea -- Preparation of liquor for use in sensory tests"

As for the actual tea making, yes, leaving the bag in longer will make a stronger cup of tea. The concentration of caffeine (along with flavour molecules and everything else) will slowly trend towards an equal concentration in the leaf and in the water. The longer you leave the tea bag/leaves in the water, the closer to equilibrium you will get.

There are other factors that affect this, such as the temperature of the water, cut of the leaves, bag versus loose leaf and so on, but the trend is always towards equilibrium as time progresses.

I'm not sure where the upper limit of this lies, but I think that once the cup is cold there's no point in it anyway. Thus, leaving the bag in for an hour is a bit much. I usually steep my tea for 3-6 minutes, depending on how strong I want it to be.