Everyone has their own ideas about how long to steep tea, but the main rule is not to "stew" the tea by letting the leaves steep forever, resulting in a bitter, unpleasant drink. Beyond that, feel free to modify these suggestions to suit your taste.
Black tea – usually steep for 3 - 5 minutes.
Green tea – generally, steep no more than 2 minutes. Many will taste best after only one minute (note: if you think you don't like green tea, it may be because you've only had it after it steeped far too long).
Oolong – steep about 2 minutes (most oolongs can be infused multiple times).
White tea – steep about 3 minutes, unless it's flavored (in which case you may want to steep it for a shorter time). White teas can also be infused more than once.
Herbal, Rooibos and Mate – steep for 5 minutes or longer. Because these "infusions" don't contain any tea (see "Types of Tea: Herbal Infusions"), they won't get bitter when steeped longer and often the benefits of the herbs are maximized by steeping for about 10 minutes.