If you mean sugar as simply sugar, the answer could be as low as "none." We do not need sugar - refined or natural, or any variation, such as honey or maple syrup in our diet at all.
But what is a maximum amount? The Zone diet would recommend a snack contain 9 grams of carbohydrate (along with 7 grams of protein and 3 of fat). Also, a small meal would be twice that, 18 grams of carbohydrate, 14 protein, and 3 fat; or a large meal, 27, 21, and 9.
But that means a large meal would only have a teaspoon (28 grams) of sugar. And it would be healthier to have carbohydrates from vegetables or whole fruit than to have sugar at all.
To learn more, take a look at my Hub, "Is there a right way to eat?"