It can vary by tea, but generally you'll be safe with these guidelines. For a standard mug (which usually holds 10 to 12 ounces), you'll just double the amount recommended for a 6 oz. cup. If you're using a teapot, it's a good idea to measure how many ounces it holds.
Herbal tea – use a heaping teaspoon per 6 oz. of water
White tea – use two teaspoons per 6 oz. of water