To make the perfect cup of tea you should generally use one heap teaspoon of tea per cup of water.
Different teas have different brewing times and preferred temperatures.
White and green teas should be brewed at around 80*C, and black around 95*C. Steeping tea for too long or using boiling water will result in more tannins being released, resulting in a bitter and more astringent tea. The amino acids responsible for teas flavour are released at a lower temperature. There are two ways to make sure your water is not too hot, either stop the kettle just before it boils, or pour boiling water into your tea cup or pot first to allow it to cool a little before adding your tea.
White tea should be steeped for 1-3 minutes, green steeped for 1-2 minutes, and black tea either 45-60 seconds (without milk), or for a stronger richer tea served with milk, 2-3 minutes.
Good quality teas can be reused. They can be infused two to three times, which will bring out different flavours and subtleties in each brew. The Chinese believe that the second or third brew of good quality tea is often the best.
Herbal teas on the other hand which can be made from flowers, leaves, branches, barks, seeds and roots should also be brewed for different lengths of time, to get maximum flavour and health benefits from your tea.
A general rule of thumb if it is a flower or leaf you can pour boiling water over your herb and then infuse it for 3-4 minutes. Harder herbs like seeds, roots and barks will produce a richer tasting tea and will draw more therapeutic properties by brewing them for longer. A decoction which is when you simmer your tea in a pot for 5-10 minutes is ideal for teas made up of these tougher herbs like that found in chai. When you have tea blends though with leaves and roots you have to find a happy medium with your brewing time.