There are WAY too many variables to put a single number on "too much sugar." The biggest of which is what else you are eating with that sugar. Second to that is what type of sugar you're talking about.
Sugar gets its bad name from it's ability to raise your blood sugar level, and as a result, your insulin level. The thing is, if you add anything to that sugar, the rate at which it raises your blood sugar level decreases. Specifically, fiber and fat are significant digestive dampeners (to coin a phrase), meaning that they slow the rate at which your body digests, and therefore absorbs, nutrients.
Make sure that you're getting some good, non-saturated fats and plenty of fiber with each meal, and then worry more about counting calories than grams of sugar. That said, sugar-added foods are a good place to start cutting calories, if need be.
None of this, of course, applies to a ketogenic diet.