This can vary significantly from tea to tea, but there are rules of thumb. In general, black tea needs to brew 3-5 minutes for best flavor. They will often get bitter if you let them brew more than 6 or 7 minutes (some much sooner). Green tea often needs only 1 minute, but 2-3 minutes is not unusual. Oolongs typically need 2-3 minutes. Herbals need 5-10 minutes, and they generally don’t get bitter, so you can let them brew as long as you like. Keep in mind that finely cut teas, whether black, green, or herbal, require shorter brewing times than large leaf teas.