The strength of the tea should be varied by changing the amount of tea leaves used, not by changing the steeping time.
 

The amount of tea to be used per amount of water differs from tea to tea, but one “perfect tea spoon” for each teacup of water (7-8 oz.).
 

Stronger teas, such as Assam, which are prepared with milk, are often prepared with more leaves.
 

The more delicate high-grown teas such as a Darjeeling are prepared with somewhat fewer leaves (as the stronger mid-flavors can overwhelm the champagne notes).