Usually I make some tea. If it's not strong enough I let it brew a bit longer. If it is delicious I call it a good brew. If it has brewed for too long then I would probably say that it has been stewing too long and is too strong.
Infusion - The process of pouring water over a substance, or steeping the substance in water, in order to impregnate the liquid with its properties or virtues (OED).
Brew - To make (ale, beer, and the like) by infusion, boiling, and fermentation (OED).
Stew - To undergo stewing; to be cooked by slow boiling in a closed vessel. Also (of an infusion of tea, etc.), to ‘stand’ on the leaves, etc. (OED).