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All teas are different, I have some that I need to brew for three times as long as others, despite looking the same!
Generally though, you want to use water at the correct temperature. Soft water helps too but temperature is the real secret. Never use boiling water to brew tea, especially green.
Your water should be no higher than 85 degrees Celsius - I use 80 - and sometimes even lower. The lower temperature brings out the teas inherent sweetness and flavours. If you use water that's too hot, or brew for to long, or (shudder) both, you will end up with something very bitter and astringent :(