I've heard anywhere from 0.75 of a cup for that batch to 2 cups -- a friend of mine does not accept that a beer will carbonate and taste better after a few weeks (sigh)
If you don't have a scale, that's about 3/4 cup. If you're using table sugar, decrease the amount by 10% or so. It's more accurate to weigh the sugar, because sugar can pack down in a cup, or be more finely granulated.