Every tea has different steeping times. Some tea makers may alert you when your tea is ready. For others, you may need to set a timer or watch the clock.
My personal rule is to go by taste. As you drink more tea, you will learn what you like and how long your favorite drinks need to steep. I prefer my tea slightly over-steeped and don't mind the slight bitterness this brings.
Others seek a light flavor, and under steeped may be the perfect match.