The best way to control the strength of your tea is the alter the amount of tea leaves you use rather than changing the brew time. People will often try to let their tea brew a long time to get a strong cup, only to find it tastes bitter. To get the best flavor, you should brew the tea according to the directions (or according to your own experiments). Then if you find the tea is too strong, try using a smaller amount of tea leaves; if too weak, try a larger amount of tea leaves.