I have been reading Dr. Walford's book on caloric restriction in most of my free time (which hasn't been much since the move). It is a good book and pretty convincing. Besides his arguments for caloric restriction, there is a lot of good information about health in general. There is even a section on the chemicals in tea. Although it is not yet proven, he points to evidence that there are a lot of benefits to green tea and recommends it for his CRON (calorie restriction with optimal nutrition) diet. About steeping he says:

It is worth mentioning that steeping tea for three to four minutes is enough to get all the antioxidants in the beverage. Prolonged steeping releases oxalates, a component in three-quarters of kidney stones.

My previous thoughts about tea steeping were "the longer the better." But now I time it to four minutes.
Also, drink jars and jars of green tea every day. He says:

Epidemiologic studies in humans remain equivocal, possibly due to dosages, since over ten cups of tea per day may be required to manifest a significant effect in humans for cancer, for cardiovascular and liver effects, for lowering of cholestrol and triglycerides, and for an increase in HDL.