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Use a timer. Some black teas are completely unforgiving. Thirty seconds additional steeping time can make the difference between a Darjeeling that’s perfect and one that’s undrinkably astringent. Large-leaf Nilgiri or
Ceylon are the most forgiving of teas and can steep a minute or two longer than ideal without appreciable harm. Most others seem to lie between these extremes. The most important variable is the size of the leaf: the larger the leaf, the longer you must steep it; the smaller the leaf, the more surface it exposes to the water and the quicker the goodness is drawn out of it. Experiment with this in mind and you will discover the optimum steeping time for the teas you use. Once you take tea-making seriously, sooner or later you will acquire a timer for the purpose.
CTC teas can sometimes steep as little as a minute. As a general rule, no black tea of Orthodox manufacture should steep less than 3 minutes. The ideal time for most Darjeelings is somewhere between 3 and 4 minutes, I think, depending on the individual tea. Three to 4 minutes is also ideal for most broken leaf teas, although many a small-leaf Ceylon tastes best at 5 minutes. Ceylon OP’s and other large-leaf teas – Formosa Oolong, for instance – are at their best after 7 minutes’ steeping. Earl Grey and Lapsang Souchong should be treated like black teas, but other scented teas may vary from under 3 minutes upwards according to taste. The rule is, the shorter the steeping time, the more aroma.
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This might hold true for black tea. I know with Pu-erh a lot of people (including well-meaning but clueless merchants) don’t know what to tell you. Most Pu-erhs benefit from multiple steepings, each being rather short. I start with 15 seconds, then 30, then 45, 60, and above that. It works for most raw or ripe Pu-erhs. Only the rare variety wants to be steeped for several minutes in the beginning despite how many people erroneously tell you to steep Pu-erh for much longer times. –Teaternity